- Swap the "build" and "gen" terms used in this repo to match
what we do in other repos. "gen" generates a bunch of files, which
is needed by "build". Unlike other repos, the output of "gen" doesn't
need to be checked in to Git however.
- The clean target now cleans the generated directory.
- Makefile.core.mk now defines SOURCE_BRANCH_NAME that decides which
branch we get reference material from. This makes it much simpler to
switch branches.
- Added the "update_all" target which does all the work needed to pull
in the imported content.