[Merge] ~codyshepherd/livecd-rootfs/+git/livecd-rootfs:buildd-bootable/xenial into livecd-rootfs:ubuntu/xenial

[Cody Shepherd]

Also, on the issue of architecture: afaik lxd tarballs _are_ built for arm64, so the buildd target will still need to be built, and as we don't want to ship or support buildd images for arm64, it seems that the current pattern of exiting the buildd hook under arm64 is likely the way we want to go.
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https://code.launchpad.net/~codyshepherd/livecd-rootfs/+git/livecd-rootfs/+merge/378975
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