[Merge] ~vorlon/livecd-rootfs:raspi-only-arm64 into livecd-rootfs:ubuntu/master

[Dave Jones]

Review: Approve

Looks good to me! There's a few other bits that need dropping too (like arm64+raspi3) but I should rebase my tidy-up branches again as they incorporate those changes (and some of these too :)
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https://code.launchpad.net/~vorlon/livecd-rootfs/+git/livecd-rootfs/+merge/457202
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