[Merge] ~xnox/livecd-rootfs:more-mounts into livecd-rootfs:ubuntu/master

[Robert C Jennings]

Review: Approve

Ah ha, I was missing /dev/mem (because things were happy on groovy, but I'll check this in focal).  The securityfs line is something I've added in my development and that's worked well.  So I'm +1 on this in theory.

The other part of this work that I'm looking at is setup of mounts in live-build/auto/build:preinstall_snaps() around the snap_validate_seed call (because I'm going to put a `snap-preseed --reset; snap-preseed` call in snap_validate_seed so we don't need to sprinkle that elsewhere.
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https://code.launchpad.net/~xnox/livecd-rootfs/+git/livecd-rootfs/+merge/387438
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~xnox/livecd-rootfs:more-mounts into livecd-rootfs:ubuntu/master.