[Merge] ~codyshepherd/livecd-rootfs/+git/livecd-rootfs:tarball-transform into livecd-rootfs:ubuntu/xenial

[Cody Shepherd]

I've built the ubuntu-base:buildd project:subproject from these recent changes and confirmed:

* the .img file, at 254 MiB, boots in multipass and contains updated package versions
* the lxd and rootfs tarballs all unzip to directory trees as expected (instead of ext4 and other artifacts as the Multipass team had seen with previous verification attempts)
* the lxd and rootfs tarballs have not grown in size
-- 
https://code.launchpad.net/~codyshepherd/livecd-rootfs/+git/livecd-rootfs/+merge/392066
Your team Ubuntu Core Development Team is subscribed to branch livecd-rootfs:ubuntu/xenial.