Merge lp://staging/~ubuntu-core-dev/livecd-rootfs/split-custom-tarball into lp://staging/livecd-rootfs

I think this looks OK with just one small inline comment, and ubuntu-cdimage and ubuntu-system-image/server should now be ready for it. What testing have you been able to do of this, if any?

We'll need to be pretty careful about landing this, though, and it's all-or-nothing right now. Stéphane had a suggestion to make this easier which I think is worth doing. Now that click-apparmor has been fixed, it should be safe to include these click packages in both rootfs and custom temporarily (obviously incurring a space penalty on devices in the interim). That would then give us the following landing procedure:

  1) land a modified version of this branch which includes packages in both rootfs and custom; don't change system-image configuration yet
  2) once step 1 is done, build an image; check that relevant packages are still usable on a device (i.e. in rootfs); inspect published custom.tar.gz by hand and make any necessary corrections
  3) modify all relevant custom tarballs built elsewhere to include the packages due to be removed from the rootfs (can be done in parallel with steps 1 and 2)
  4) once all of steps 1, 2, and 3 are done, switch system-image configuration to use cdimage-custom
  5) once step 4 is done, build an image; check that relevant packages are still usable on a device, and that they are present in /custom
  6) once step 5 is done, land a further livecd-rootfs change that drops the packages from rootfs
  7) once step 6 is done, build an image; check that relevant packages are still usable on a device, and that they are only present in /custom and not rootfs

As per Joe (product owner) we won't be pre-installing sudoku, dropping letters, filemanager, and terminal on commercial builds. My feeling then is that they should be removed from here.

The point of this work was to decouple things we aren't installing in
commercial builds from the set of things that go in the community
images. It's not clear why what we install in commercial builds should
continue to have a bearing here. We're removing them from the rootfs so
that the two sets of builds can have independent click package
selections.

I think this is working properly now with my modifications, and should be incrementally-landable.