grubby-bls: also print the absolute path in the --default-kernel option
    
    The absolute path to the kernel and initramfs images is printed by the
    --info option, but it's not printed by the --default-kernel option.
    
    Resolves: rhbz#1649778
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>