grubby-bls: set saved_entry to id instead of title on set-default
    
    The grubby wrapper sets the saved_entry in grubenv to the entry title
    field since that's done by grubby. But for BLS the id is the filename
    without the .conf extension, this saved_entry is set to this when a
    new kernel is installed.
    
    So for consistency also use the entry id for the --set-default option.
    
    Related: rhbz#1638103
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>