Make mk2rbc accept either a makefile or product

The current behavior of mk2rbc is that it can accept either
a makefile to convert, or if it's also generating a launcher,
it must accept a product name instead of a makefile.

For board configuration, we need to convert a specific makefile,
but we also want to generate a launcher. Change mk2rbc so that
its non-flag arguments can be either makefiles or product names.

Bug: 201700692
Test: ./build/bazel/ci/rbc_product_config.sh aosp_arm64-userdebug
      with board config changes patched in
Change-Id: I521e31fe0bdc608b8f26c9aa803ca690dd1c538e