workspace: Adds support for Bazel facades/backends

This makes use of Bazel's configuration feature 'label_flag' to create
a set of configurations that have overridable defaults from the command
line.

No-Docs-Update-Reason: Docs further up the stack in 41561.
Change-Id: Id4064c8f11bc6ec468aad18ef4d02fa472fdc590
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/41560
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Akira Baruah <akirabaruah@google.com>
Commit-Queue: Keir Mierle <keir@google.com>
diff --git a/WORKSPACE b/WORKSPACE
index b87d7dd..a34cacf 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -215,3 +215,11 @@
     strip_prefix = "buildtools-4.0.1",
     url = "https://github.com/bazelbuild/buildtools/archive/4.0.1.tar.gz",
 )
+
+load("//pw_build:target_config.bzl", "pigweed_config")
+
+# Configure Pigweeds backend.
+pigweed_config(
+    name = "pigweed_config",
+    build_file = "//targets:default_config.BUILD",
+)