Use vmdebootstrap-0.8

Switch to vmdebootstrap-0.8 and refresh the btrfs patch. Debian bug #786767 (i386 arch kernel package not found) is fixed in vmdebootstrap 0.8.

I checked that I could build images for each target, except for raspberry (still hangs during kernel update). I also checked that the virtualbox image could boot, but couldn't test much more due to the network interface renaming issue.