Redundant creation of oem user in rpi desktop images

There are two (semi-related) issues which are currently preventing the creation of Ubuntu desktop images for the Raspberry Pi:

The first is that livecd-rootfs is erroneously trying to create the oem user (and run oem-config-prepare) twice in live-build/ubuntu/hooks/099-ubuntu-image-customization.chroot.

The second is that the ubuntu-raspi-settings-desktop package is attempt to create the oem user and run oem-config-prepare in its postinst. This is the wrong place to accomplish this as there's no guarantee that oem-config-prepare is installed at the time that ubuntu-raspi-settings-desktop is installed. It cannot be a dependency because oem-config is uninstalled after first boot.

It's likely that the *correct* place to do this configuration is in neither of these places, but in oem-config's postinst (along with some check that it's being installed in an image that's never been booted, e.g. check for the existence of some sentinel created on first boot ... maybe /var/log/boot.log or something). Still, that fix can wait until it's not release week!