[Merge] ~dbungert/livecd-rootfs:minimal+tpmfde into livecd-rootfs:ubuntu/master

[Steve Langasek]

Review: Needs Fixing

Thanks.  With Desktop approving of the direction, this looks good wrt implementation, but incomplete.

live-build/ubuntu/hooks/020-ubuntu-enhanced-sb.binary needs updated to do its thing for both of the enhanced-secureboot squashfs layers.
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https://code.launchpad.net/~dbungert/livecd-rootfs/+git/livecd-rootfs/+merge/451562
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