[Merge] lp:~rbalint/livecd-rootfs/deps-xenial into lp:~ubuntu-core-dev/livecd-rootfs/xenial-proposed

[Balint Reczey]

The autopkgtest tests cover all changes except the pxz->xz change which needs a test build for armhf:raspi2.

While the change affects several lines I think all of them are straightforward and safe and are also easy to follow in the separate commits.
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https://code.launchpad.net/~rbalint/livecd-rootfs/deps-xenial/+merge/333132
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