Add RBC board configuration

This does largely the same thing as in product configuration,
it converts the board makefiles and evaluates them in place
of an $(include).

Bug: 201700692
Test: Temporarily replace the ifndef RBC_BOARD_CONFIG with RBC_PRODUCT_CONFIG,
      then ./build/bazel/ci/rbc_product_config.sh aosp_arm64-userdebug
Change-Id: I91c3dd6b91cf6bfeb18a5fff95d53b7a2c113c57