[checks] Relax check for isohybrid command
    
    It is used only in createiso and productimg phases.
    
    For productimg, it needs to be present only when the compose is bootable
    and productimg phase is explicitly enabled.
    
    For createiso, it is only needed if runroot is not enabled.
    
    Additionally, if we detect pungi running on arch for which syslinux is
    not available, a warning is printed, but the compose is allowed to
    continue (and possibly crash later).
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>