Return product config variables from board_configuration

The board config can actually change a few product config
variables that are not made read-only.

Bug: 201700692
Test: ./build/bazel/ci/rbc_regression_test.sh -b aosp_cf_x86_64_phone
Change-Id: Ib2ac04ac2c5a56d32bca51fc8189e44e180075ee