grubby-bls: fix --add-kernel not working when using the --args option
    
    The script prints an error if --args option is used without --update-kernel
    but it's also valid to use it with the --add-kernel option.
    
    Resolves: rhbz#1691004
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>