Strip more board config variables

These variables' values show up in the command line
of certain build commands, so they need to be stable
to have stable ninja files.

The starlark board configuration strips these variables,
causing a discrepency between the starlark and make
versions of board configuration.

Bug: 201700692
Test: ./build/bazel/ci/rbc_regression_test.sh beagle_x15-userdebug
Change-Id: Id053435409821a3fe5997c07610ef835e0c83112