When we rebuild an image with changed local archives,
we would preferably remove the already existing preferences files.
However, there's no way to make sure that we remove enough or too
much, hence we don't remove any at all and stay with the dogma:
"If you change something in the config tree affecting $stage,
you need to rebuild that stage from scratch."
So for pinning changes for local archives, this means, since it's
affecting the chroot stage, we'll have to rebuild the chroot stage:
"lb clean --chroot && lb chroot"
Previously, we needed to keep the lb_* prefix as the scripts
could also be executed standalone. Since the lb wrapper is
mandatory since squeeze, we can drop this now.
Daniel Baumann