Start build target config files

This change defines the build target system used by Pigweed, where the
build is configured using per-target globals imported from a target .gni
file. A single build arg specifying the path to the target file replaces
the previous args.

A default build variables file is provided in the Pigweed root. This
file defines and documents all of the build variables used by Pigweed.

A top-level targets directory is added to define the targets used by
Pigweed developers. Currently, only a "host" target is added, which
configures the build for a development system based on the host OS.

Change-Id: Ie62b8d41e5789bfe7fffe20462160142c79076c4
diff --git a/BUILDCONFIG.gn b/BUILDCONFIG.gn
index 0a2648f..d97d96c 100644
--- a/BUILDCONFIG.gn
+++ b/BUILDCONFIG.gn
@@ -37,18 +37,15 @@
 }
 
 declare_args() {
-  # The name of the GN template used to build Pigweed executables.
-  pw_executable_target_type = "executable"
-
-  # Path to the .gni file in which the pw_executable_target_type template is
-  # defined. Empty string if using a GN primitive.
-  pw_executable_target_path = ""
-
-  # Toolchain for the current build target (desktop/device).
+  # Path to the Pigweed variables config file for the build target.
   #
-  # If this is changed, you must run `ninja -t clean` to remove any files from
-  # the previous toolchain's build before building again.
-  pw_target_toolchain = "$dir_pw_toolchain:x86_linux_o2"
+  # When this is changed, you must run `ninja -t clean` to remove any files from
+  # the previous target's build before building again.
+  pw_target_config = "$dir_pigweed/targets/host/host.gni"
 }
 
+import(pw_target_config)
+
+assert(pw_target_toolchain != "",
+       "Build target must provide its own toolchain.")
 set_default_toolchain(pw_target_toolchain)
diff --git a/pw_build/pw_executable.gni b/pw_build/pw_executable.gni
index 9b8666d..b961803 100644
--- a/pw_build/pw_executable.gni
+++ b/pw_build/pw_executable.gni
@@ -15,16 +15,7 @@
 # Wrapper for Pigweed executable build targets which uses a globally-defined,
 # configurable target type.
 template("pw_executable") {
-  assert(defined(pw_executable_target_type),
-         "build argument pw_executable_target_type must be defined")
-  assert(defined(pw_executable_target_path),
-         "build argument pw_executable_target_path must be defined")
-
-  if (pw_executable_target_path != "") {
-    import(invoker.pw_executable_target_path)
-  }
-
-  target(invoker.pw_executable_target_type, target_name) {
+  target(pw_executable_config.target_type, target_name) {
     forward_variables_from(invoker, "*")
   }
 }
diff --git a/pw_vars_default.gni b/pw_vars_default.gni
new file mode 100644
index 0000000..f0fd89f
--- /dev/null
+++ b/pw_vars_default.gni
@@ -0,0 +1,46 @@
+# Copyright 2019 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+# This file defines default values for all of the build variables used by
+# Pigweed.
+#
+# Each individual variable can be overriden by a target to configure the system
+# to build for it. This is done in a .gni file specific to the target by first
+# importing this file, then changing the variables as desired.
+
+# Options which configure the executable targets created in Pigweed builds.
+pw_executable_config = {
+  # The name of the GN target type used to build Pigweed executables.
+  #
+  # If this is a custom template, the .gni file containing the template must
+  # be imported at the top of this file to make it globally available.
+  target_type = "executable"
+
+  # Path to an instantiation of the "linker_script" GN template defining the
+  # linker script configuration for the target.
+  linker_script_target = ""
+
+  # Path to the Bloaty configuration file that defines the memory layout and
+  # capacities for the target binaries.
+  bloaty_config_file = ""
+}
+
+# Default toolchain for the build target.
+#
+# If this is changed, you must run `ninja -t clean` to remove any files from
+# the previous toolchain's build before building again.
+pw_target_toolchain = ""
+
+# Implementation of a main function for "pw_test" unit test binaries.
+pw_unit_test_main = "$dir_pw_unit_test:main"
diff --git a/targets/host/host.gni b/targets/host/host.gni
new file mode 100644
index 0000000..33806be
--- /dev/null
+++ b/targets/host/host.gni
@@ -0,0 +1,24 @@
+# Copyright 2019 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+# Config file for a host desktop system.
+if (host_os == "linux") {
+  import("linux.gni")
+} else if (host_os == "mac") {
+  import("macos.gni")
+} else if (host_os == "win") {
+  import("windows.gni")
+} else {
+  assert(false, "Please define a host config for your system: $host_os")
+}
diff --git a/targets/host/linux.gni b/targets/host/linux.gni
new file mode 100644
index 0000000..f1d7a3c
--- /dev/null
+++ b/targets/host/linux.gni
@@ -0,0 +1,17 @@
+# Copyright 2019 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+import("$dir_pigweed/pw_vars_default.gni")
+
+pw_target_toolchain = "$dir_pw_toolchain:x86_linux_o2"
diff --git a/targets/host/macos.gni b/targets/host/macos.gni
new file mode 100644
index 0000000..a4a5662
--- /dev/null
+++ b/targets/host/macos.gni
@@ -0,0 +1,15 @@
+# Copyright 2019 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+import("$dir_pigweed/pw_vars_default.gni")
diff --git a/targets/host/windows.gni b/targets/host/windows.gni
new file mode 100644
index 0000000..a4a5662
--- /dev/null
+++ b/targets/host/windows.gni
@@ -0,0 +1,15 @@
+# Copyright 2019 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+import("$dir_pigweed/pw_vars_default.gni")