Bug#549681: bug#549681: mkvmlinuz: use xz to compress vmlinuz-boxed initrd

> some OpenFirmware implementations, such as the one in the PegasosII,
> have a 12 MB size limit on kernel images, and no initrd loading
> capability. The latter is worked around by merging the initrd into the
> image with the "mkvmlinuz" tool, however the generated images are
> unbootable if they exceed 12 MB.

The attached patch brings vmlinuz from about 13MB to about 9.5MB, which
is well under the 12MB limit. Downside is that xz compressing is
noticeably slower than currently used gzip, but decompressing speed
difference is not noticeable.

I've tested it on Pegasos II machine, it boots really fast. Updated
mkvmlinuz package is waiting on mentors.debian.net for a willing sponsor
for upload.

Milan