20-grub-install: Use relative paths for btrfs in BLS snippets
    
    When SUSE_BTRFS_SNAPSHOT_BOOTING=true is set in /etc/default/grub, paths
    to kernel and initrd images need to be relative. Since are used along with
    snapper, configured so the default btrfs subvolume is the current snapshot.
    
    Resolves: rhbz#1906191