Add support for GRUB patches from SUSE

Some GRUBs out there (Fedora, openSUSE) have an option that makes all
paths relative to the default subvolume of the filesystem. This can be
used to include /boot in your snapshots and roll them back without
having to regenerate grub.cfg.

However, enabling that option will break grub-btrfs, because loading the
kernel from a different snapshot requires the paths to be absolute.

To make this work, GRUB has to be told explicitly to access the root
subvolume when booting to a snapshot.

41_snapshots-btrfs

@@ -224,7 +224,12 @@ make_menu_entries()
             search --no-floppy --fs-uuid  --set=root ${boot_hs} ${boot_uuid}
         else
             search --no-floppy --fs-uuid  --set=root ${boot_uuid}
+        fi"
+        if [ "${SUSE_BTRFS_SNAPSHOT_BOOTING:-"false"}" = "true" ]; then
+            entry "\
+        set btrfs_subvolid=5"
         fi
+        entry "\
         echo 'Loading Snapshot: "${snap_date_trim}" "${snap_dir_name_trim}"'
         echo 'Loading Kernel: "${k}" ...'
         linux \"${boot_dir_root_grub}/"${k}"\" root="${LINUX_ROOT_DEVICE}" ${kernel_parameters} ${rootflags}subvol=\""${snap_dir_name_trim}"\""
@@ -259,7 +264,12 @@ make_menu_entries()
             search --no-floppy --fs-uuid  --set=root ${boot_hs} ${boot_uuid}
         else
             search --no-floppy --fs-uuid  --set=root ${boot_uuid}
+        fi"
+        if [ "${SUSE_BTRFS_SNAPSHOT_BOOTING:-"false"}" = "true" ]; then
+            entry "\
+        set btrfs_subvolid=5"
         fi
+        entry "\
         echo 'Loading Snapshot: "${snap_date_trim}" "${snap_dir_name_trim}"'
         echo 'Loading Kernel: "${k}" ...'
         linux \"${boot_dir_root_grub}/"${k}"\" root="${LINUX_ROOT_DEVICE}" ${kernel_parameters} ${rootflags}subvol=\""${snap_dir_name_trim}"\""