Initial Pigweed modules

This change adds Pigweed's first few (incomplete) modules: pw_build,
pw_status, and pw_toolchain. The main scaffolding for Pigweed's GN-based
build system is also included, with a root BUILDCONFIG and BUILD.gn file
defining a top-level modules build target.

Change-Id: I09599443d50696343d6c5b85853cb5cee3ce0026
diff --git a/.gn b/.gn
new file mode 100644
index 0000000..de099cc
--- /dev/null
+++ b/.gn
@@ -0,0 +1,16 @@
+# 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.
+
+buildconfig = "//BUILDCONFIG.gn"
+script_executable = "python3"