Code review comment for ~waveform/ubiquity:prepare-package

I'm told the plan is for subiquity to grow this functionality, but I've no details on delivery (which makes me at least want a backup plan in place). Incidentally, this isn't new code as such: it's code from livecd-rootfs that should've been in oem-config all along. One of the merges in the associated ticket [1] removes that code from livecd-rootfs, this merge places it in an extra package here, and then the final merge from the ticket [2] uses this package in the image definition yaml (which is currently missing all the user setup stuff).

[1]: https://code.launchpad.net/~waveform/livecd-rootfs/+git/livecd-rootfs/+merge/456463

[2]: https://code.launchpad.net/~waveform/ubuntu-images/+git/ubuntu-images/+merge/456465