[Merge] livecd-rootfs:default-compat into livecd-rootfs:ubuntu/master

[Paride Legovini]

Review: Needs Information

+1 modulo John Chittum's comments.

Just one comment/question: should we pin compat=1.1 to we keep building images compatible with all the supported Ubuntu releases even if qemu upstream changes the default image format?
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https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/427296
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