unified-iso: Only link to non-empty variants
    
    Instead of adding images to metadata and then creating hardlinks in a
    separate step, do it immediately while we still have accurate
    information about what variants it should be linked to.
    
    Fixes: https://pagure.io/pungi/issue/670
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>