Remove hardcodings of the git repo path

There was already a $git_repo variable for this, use it.
Don't hardcode /live/image everywhere
Upgrade live-boot

Inst live-boot [3.0~a35-1] (3.0.1-1 Debian:7.5/stable [all])
Re-apply gitfs patches to live-boot

This basically applies:

	git diff 042cac9 2ee06be --  lib/live/boot* | sudo patch -p1

However, the filenames were changed, so the new files needed to be
given to the patch prompts.

This commit breaks the boot, due to some actual changes in live-boot. This will
be fixed in the next commit.
Update git-fs to work with newer live-boot

The overlay script in live-boot got changed subtly, so our git-fs additions
were now setting the wrong variable. This makes booting work again.
Update initramfs

This includes the live-boot upgrade
Delete CUPS cache file
Update path to live boot medium

With the updated live-boot, the place where the boot medium is mounted
has changed to from /live/image to /lib/live/mount/medium.
Make /.git work again with updated live-boot

Some paths changed in the updated live-boot, which prevented /.git from
being properly setup read-write after booting.
Clean up install-to-disk.sh

This shouldn't change any behaviour, it just puts some more code inside
a function, to make things a bit more clear.