CI: use boot2container instead of full distro under qemu
Demo of freedesktop/ci-templates!105 (merged)
We now build a new kernel image, store it on the generic package registry, and then use it to directly use boot2container instead of a full distro under qemu.
Keeping this as a draft as long as the ci-templates bit is not merged and we can remove the overrides
Edited by Benjamin Tissoires