Reapply "Add support for product-specific build flags"

1. release config maps now specify where the flag definitions are found.
2. PRODUCT_RELEASE_CONFIG_MAPS specifies additional release config map
files to use.

This allows product config to specify build flags, which can then be
specified by users of that product.

Bug: b/302593603, b/309477343
Test: manual
Change-Id: Ic1f0512ec4b06ac94dd3f29eadd6a03ba8ebf6d2