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
Test: manual
Change-Id: I660a3d88c2aaecc14d6f370bebb0d05a8cc224f2