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>