libbe(3)/bectl(8): Kill off the 'add' functionality for now

The mostly-undocumented 'add' functionality, from initial read-through, is
intended for construction of deep ("bdrewery style") boot environments.
However, it's mostly broken at this point. #if SOON it out on both sides
so that we're not exposing a broken API/feature.

Work will resume on it in due time.