grubby-bls: allow to specify the same kernel param multiple times
    
    The kernel command line can contain parameters that are specified multiple
    times. This is supported by the old grubby tool but ins't supported by the
    grubby BLS wrapper. Allow the grubby wrapper to do the same.
    
    Resolves: rhbz#1652486
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>