add(resize_btrfs): Add temporary script to resize btrfs on boot.

Eventually this functionality will be moved into the `cgpt resize`
command but since the best way to handle btrfs is very different from
ext4 I don't want to support both in the cgpt code at the same time.
Merge pull request #56 from marineam/btrfs

add(resize_btrfs): Add temporary script to resize btrfs on boot.