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>