Remove cow dir contents that are in the way for custom mounts.

Remove cow dir contents that are in the way for custom mounts.

This is mostly relevant if you activate persistence post boot; in that
case you (or live-config) may have modified or added some file, which
makes it appear in the cow dir. If you later activate a persistent
volume which contains that file, the old file will "block" the one
from the persistent volume, which likely isn't what you expect.