oem-config-prepare needs manual execution

At present, in pre-installed images that rely on oem-config for their first-boot configuration (the Raspberry Pi Ubuntu desktop images are probably the most obvious of these), it is not enough to simply include oem-config (and an appropriate front-end) in the image. The oem-config-prepare script must also be run, and the appropriate oem user (with the correct UID) must also be created.

Currently this is handled by a series of hacks in either livecd-rootfs or ubuntu-image (whichever is being used to build the image). In the case of livecd-rootfs [1]:

  # Create the oem user account only if it doesn't already exist
  if ! id "oem" &>/dev/null; then
    /usr/sbin/useradd -d /home/oem -G adm,sudo -m -N -u 29999 oem
    /usr/sbin/oem-config-prepare --quiet
    touch "/var/lib/oem-config/run"
  fi

This is doubly pointless since now we use ubuntu-image rather than livecd-rootfs for building these images anyway. Speaking of ubuntu-image, other there we do this [2]:

  # FIXME: Temporarily, we also need to make sure that for desktop pi images
  # we actually run oem-config-prepare before first boot. Otherwise we won't
  # be able to boot into oem-config properly.
  # This should be done via a package postinst ideally, so this manual
  # customization should be only temporary.
  manual:
    execute:
      - path: "/usr/sbin/oem-config-prepare"

(this is also wrong since it misses the creation of the oem user).

It would be much cleaner in both cases to have an extra package (appropriately named "oem-config-prepare") that simply depends on the "oem-config" package, which contains the oem-config-prepare script, but carries out these steps in its postinst. Thus the ubuntu-image yaml can simply include this package under "extra-packages" and the livecd-rootfs hook lines can disappear (in fact the entire hook can disappear as it's for pre-installed desktops which we don't make with livecd-rootfs at all).

[1]: https://git.launchpad.net/livecd-rootfs/tree/live-build/ubuntu/hooks/099-ubuntu-image-customization.chroot#n14

[2]: https://git.launchpad.net/ubuntu-images/tree/ubuntu-pi-arm64.yaml#n68