Revert "kernel: change compression of pae kernel to bz2."

This reverts commit 3e8286210f4e167c9ad2bb20b7fe3a56d2435b52.

bz2 is not supported by debian xen. So users that have problems
with xz compression need to update the pygrub on the host.
Merge branch 'next' of ssh://git.ipfire.org/pub/git/ipfire-2.x into next