dmsquash: Add squashfs support to rd.live.fsimg

dmsquash: Add squashfs support to rd.live.fsimg

Previously rd.live.fsimg only supported filesystems residing in
(compressed) archives.
Now rd.live.fsimg can also be used when a squashfs image is used.
This is achieved by extracting the rootfs image from the squashfs and
then continue with the default routines for rd.live.fsimg.
In addition some code duplication got removed and some documentation
got added.

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>