Add support for liveimg-squashfs
    
    This patch adds support to build squashfs images which are compatible to
    anaconda's liveimg directive and dracuts liveimg feature.
    The feature is pretty simple, as it just reuses the raw diskimage, and
    strips the disklabel, to retrieve the plain filesystem image.
    
    Two open issues:
    - Stripping the disk label is a pretty hack
    - Database schema upgrade is missing
    
    Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>