Shortening kernel and initrd filenames in /live again similar to what we did with in lenny.

Unfortunately, syslinux as of wheezy and newer apparently does not support
long filenames anymore. Therefore, we do have to shorten the filenames
from:

	/live/vmlinuz-3.2.0-1-amd64
	/live/initrd.img-3.2.0-1-amd64

to:

	/live/vmlinuz
	/live/initrd.img

In case more than one kernel flavour is used, the files are being
numbered, starting with vmlinuz1 and initrd1.img.