Ubuntu: fix bootorder on ppc64el

Much like RH/SUSE, with Ubuntu supported on ppc64, we are seeing the same boot order issues -- that is the install process is overwriting the boot order to be disk-first, regardless of the initial settings in SMS. This series fixes this up on Ubuntu ppc installs by adding a nochroot'd late script to run in the preseed, which restores the value saved in the early script. We must run nochroot, as the early script runs before /target is configured. We could, technically, run a simply cp command nochroot, then run the snippet in the chroot'd late script, but that seems unnecessarily complicated, and it also seems good to have an example of how to run late commands not-chroot'd.