fs/ext2: explicitly set a NULL UUID

"tune2fs -U clear" creates an invalid filesystem, that fsck.ext2
whines about later:

    $ make rootfs-ext2
    [--SNIP--]

    $ ./host/usr/sbin/fsck.ext4 images/rootfs.ext2
    e2fsck 1.42.7 (21-Jan-2013)
    Filesystem did not have a UUID; generating one.
    images/rootfs.ext2: clean, 4616/5120 files, 53171/131072 blocks

    $ ./host/usr/sbin/fsck.ext4 -f images/rootfs.ext2
    e2fsck 1.42.7 (21-Jan-2013)
    One or more block group descriptor checksums are invalid.  Fix<y>? yes
    Group descriptor 0 checksum is 0x4131, should be 0x8bdb.  FIXED.
    [--SNIP--]

So we set an explicitly NULL UUID instead.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

fs/ext2/genext2fs.sh

@@ -70,11 +70,14 @@ e2tunefsck() {
            "${IMG##*/}" "${GEN}"
 
     # e2fsck will force a *random* UUID, which is bad
-    # for reproducibility, so we do not want it.
+    # for reproducibility, so we do not want it. Asking
+    # tune2fs to 'clear' the UUID makes for an invalid
+    # fs, so we explicitly set a NULL UUID, which works.
     # Remove count- and time-based checks, they are not welcome
     # on embedded devices, where they can cause serious boot-time
     # issues by tremendously slowing down the boot.
-    tune2fs -U clear -c 0 -i 0 "${IMG}"
+    tune2fs -U 00000000-0000-0000-0000-000000000000 "${IMG}"
+    tune2fs -c 0 -i 0 "${IMG}"
 }
 
 # Check we know what generation to generate