Define $(PRODUCT_OUT) before running the board config

Certain board configurations reference $(PRODUCT_OUT)
through deferred expansion, which will no longer work
after conversion to starlark.

Bug: 201700692
Test: build/bazel/ci/rbc_regression_test.sh -b yukawa-userdebug
Change-Id: I02055f5c4a05e540c1752d5964d4db4306292c3b