release_config.mk: simplify initial release-config run

Additionally:
- leave a file that can be used with --maps-file to run build-flag or
  release-config again.
- leave a file containing the hash of all the release config files that
  were used, so that Soong modules can detect the need to rebuild.  This
  replaces _used_files.

The release-config binary now reads *all* release config related files
from all of the map locations we give it, and then compiles whatever is
needed to generate the active release config.

Bug: b/429239584
Test: Manual, TH
Change-Id: Ia1296301ef2ce6d990ed7dc6dbedb3342b3397a2