Try and workaround IoT rebase problems with F37 read-only change
    
    The change to read-only sysroot for IoT in F37 causes problems
    with this rebase test. It's not supported to rebase from an RO
    release (37 or 38) to a non-RO release (36). So we need to make
    sure we don't try and do that. This uses some quick hack logic,
    but it should be OK and sufficiently specific not to break
    anything even if we forget to remove it in future.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>