Bug 897352 - [gaia build system] Transition to a build_stage for assembling apps

Conflicts:
	Makefile
	apps/communications/build/build.js
	build/copy-build-stage-manifest.js
	build/test/integration/distribution.test.js
	build/utils-xpc.js
	build/utils.js
	build/webapp-manifests.js
Merge pull request #17510 from yurenju/stage3

Bug 897352 - [gaia build system] Transition to a build_stage for assembl... r=@ochameau