Move image build process to launchpad

[tobhe]

To have as little maintenance overhead and get as close to the default Ubuntu images as possible we really should be using launchpad to build images via the +LiveFS feature instead of rolling our own debootstrap scripts.

[tobhe]

Some initial work on this was done here: https://launchpad.net/~tobhe/+livefs/ubuntu/lunar/ubuntu-asahi
It uses https://code.launchpad.net/~tobhe/+git/livecd-rootfs to build working asahi squashfs and ubuntu-image targets. The scripts/lp-disk directory is an initial try to generate asahi-installer compatibel images from the squashfs produced by launchpad.

The documentation situation is a bit lacking so we should try to write our own and get it published on the Ubuntu wiki or launchpad itself. The best knowledge source for me was looking at the https://launchpad.net/~ubuntu-concept git repos

[tobhe]

Closing this issue because we have been using launchpad builds exclusively for a while now.