Wait for loop device to settle

This fixes the error posted here https://github.com/xbianonpi/xbian/issues/566 and thereby also fixes the xbian-copy feature on the Hummingboard.
Merge pull request #34 from CurlyMoo/patch-2

btrfs-auto-snapshot: Wait for loop device to settle
Fix 288cfda2e37b85834ab05c9a29b65f73b64e7a49