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.

This reverts commit 75bfc37e.

Bug: b/302593603
Test: manual
Change-Id: I031a00459893644d7f67b63b982db9ae9015ae4d