support for kernel 2.6.36-grml[64], drop backwards compatibility support, adjust...

support for kernel 2.6.36-grml[64], drop backwards compatibility support, adjust depends, update docs

The backwards compatibility stuff to support different kernel
versions with different squashfs-tools and all the mksquashfs
options used to be a ugly mess.

Now having xz support (which corresponds to what we have with
squashfs-lzma nowadays) in squashfs-tools (>=1:4.1-1) in Debian
and using a recent ondisk layout for squashfs-lzma in our kernel
versions since 2.6.35-grml[64] gives us the option to clean up
with this mess.

If custom mksquashfs binaries/options are still needed to
remaster old Grml releases or custom kernel versions either use
an older grml-live version or adjust SQUASHFS_BINARY and
SQUASHFS_OPTIONS in grml-live configuration accordingly.

grml-live depends on recent squashfs-tools and fai* versions now.