Floss: Add protobuf build to btmanagerd

Prepares build system to support protobuf with powerd D-Bus integration.

Bug: 224606285
Tag: #floss
Test: Manual - build.py

Change-Id: I9b701a84d11dde3ed9414711a371707106f28b57
diff --git a/build.py b/build.py
index 2bffdc3..b32c5eb 100755
--- a/build.py
+++ b/build.py
@@ -246,6 +246,7 @@
         self.env['CARGO_HOME'] = os.path.join(self.output_dir, 'cargo_home')
         self.env['RUSTFLAGS'] = self._generate_rustflags()
         self.env['CXX_ROOT_PATH'] = os.path.join(self.platform_dir, 'bt')
+        self.env['CROS_SYSTEM_API_ROOT'] = os.path.join(self.platform_dir, 'system_api')
 
     def run_command(self, target, args, cwd=None, env=None):
         """ Run command and stream the output.
@@ -606,6 +607,7 @@
         # Symlink things
         symlinks = [
             (os.path.join(self.git_dir, 'platform2', 'common-mk'), os.path.join(self.staging_dir, 'common-mk')),
+            (os.path.join(self.git_dir, 'platform2', 'system_api'), os.path.join(self.staging_dir, 'system_api')),
             (os.path.join(self.git_dir, 'platform2', '.gn'), os.path.join(self.staging_dir, '.gn')),
             (os.path.join(self.bt_dir), os.path.join(self.staging_dir, 'bt')),
             (os.path.join(self.git_dir, 'rust_crates'), os.path.join(self.external_dir, 'rust')),