If rootfs is a symbolic link but not to a block device, then do a separate
rm of its contents.  We have to do this because, out of cowardice, we call
rm with --one-filesystem.

Removing the '-o -h $rootdev' is ok, because if $rootdev is a symbolic
link to a block device (including lvm blockdev) then -b will still return
true.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>