release_config: better default map paths

- Automatically determine the top of the workspace.
- build-flag defaults to using `get_build_var` to get product specific
  release config maps.
- release-config defaults to using PRODUCT_RELEASE_CONFIG_MAPS but does
  not use `get_build_var` unless the argument is given.

Bug: 328495189
Test: manual
Change-Id: I4ba3c5dfab43c4ebc3eeda13318f42e886dada4e