[Merge] ~mwhudson/livecd-rootfs/+git/livecd-rootfs:tpm-seed-in-live-layer into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

So I rebased this onto ubuntu/master and it still doesn't work. I think we need two things to change to make this work:

1) https://github.com/canonical/snapd/commit/f90d9ddb0f33d4bfbae25d4fa6420332c3aa6a1c needs to be in stable (currently not in any tag)

2) we need to switch the gadget used by tpm fde systems to one that has the system-seed role (rather than just system-seed-null as the one we use today has) -- the first thing to try here is probably switching to the gadget used for UC24 as I think all the differences are rather non-essential now.
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https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/466202
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