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>

cli/koji

@@ -5353,7 +5353,8 @@ def handle_image_build(options, session, args):
     """Create a disk image given an install tree"""
     formats = ('vmdk', 'qcow', 'qcow2', 'vdi', 'vpc', 'rhevm-ova',
                'vsphere-ova', 'vagrant-virtualbox', 'vagrant-libvirt',
-               'vagrant-vmware-fusion', 'docker', 'raw-xz')
+               'vagrant-vmware-fusion', 'docker', 'raw-xz',
+               'liveimg-squashfs')
     usage = _("usage: %prog image-build [options] <name> <version> " +
               "<target> <install-tree-url> <arch> [<arch>...]")
     usage += _("\n       %prog image-build --config FILE")