There's a desire to hide Portage dependency complexity around chromeos-config-bsp from partners who are doing code changes to support model configuration such that--no matter what subsystem they are working on--one script invocation is guaranteed to produce a 100% reproducible result.

One reason that changes to ./build_packages havn't had this property, historically, is that it optimizes for performance over correctness. Because we know that chromeos-config-bsp is near the leaves of the dependency tree with relatively few reverse dependencies, we can simply guarantee that all reverse dependencies of chromeos-config-bsp are always rebuilt with --nousepkg regardless of any modified logic with relatively little additional cost.

The src/scripts/ directory is probably the right place for this. This would, in effect, be a generalization of the script that was checked-in here: https://chrome-internal-review.googlesource.com/c/chromeos/overlays/overlay-coral-private/+/449755