[Merge] ~gjolly/livecd-rootfs:feat/functions/snap_validate_seed_kern_param into livecd-rootfs:ubuntu/master

[Philip Roche]

Review: Disapprove

A change like this has already landed @ https://code.launchpad.net/~philroche/livecd-rootfs/+git/livecd-rootfs/+merge/453306 as part of addressing https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/2038957 
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https://code.launchpad.net/~gjolly/livecd-rootfs/+git/livecd-rootfs/+merge/452451
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