Setup sample_project structure

- build_overrides/pigweed.gni declares new modules.
  e.g. $dir_pw_board_led
- Copy sample_project/workshop/{string,rpc} examples to applications/

Change-Id: Ia136853a382ab5f24d2dc269ee9bf22e2d855a7b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/35000
Commit-Queue: Anthony DiGirolamo <tonymd@google.com>
Pigweed-Auto-Submit: Anthony DiGirolamo <tonymd@google.com>
Reviewed-by: Joe Ethier <jethier@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..317cd91
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,6 @@
+[submodule "third_party/pigweed"]
+	path = third_party/pigweed
+	url = https://pigweed.googlesource.com/pigweed/pigweed
+[submodule "third_party/nanopb"]
+	path = third_party/nanopb
+	url = https://pigweed.googlesource.com/third_party/github/nanopb/nanopb