Summary:
Merges the Facebook-internal Buck target definitions in `Libraries/` into the BUCK file at the root of the repo (which is currently not synced to GitHub at all). This does not affect how React Native is built in open source.
Changelog: [Internal]
Reviewed By: GijsWeterings
Differential Revision: D27967499
fbshipit-source-id: 39c51a544b3868242598072d24cb6cfb5a6e2d8c