Create build UUID

This creates a build UUID and stores it inside the chroot in
/etc/live/uuid.conf and in the binary image in .disk/live-uuid.

Together with supporting code already present in live-boot this
can be used to couple an image and it's initrd(s).

To fully work this needs a small patch in the initramfs hook in
live-boot.
Update changelog for version 3.0.5-1+lernstick2