[Merge] ~cjwatson/livecd-rootfs:xenial/exclude-makedev-properly into livecd-rootfs:ubuntu/xenial

[Adam Conrad]

Review: Approve

+1 for "This time I actually tested."
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https://code.launchpad.net/~cjwatson/livecd-rootfs/+git/livecd-rootfs/+merge/373141
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