[Merge] lp:~rcj/livecd-rootfs/trunk into lp:livecd-rootfs

[Robert C Jennings]

This can be simplified to always overwrite the assertion if it exists.  live-build/auto/build calls snap_prepare so we can drop the snap_prepare from snap_preseed.  If a hook wants to specify a different model they can call snap_prepare with a custom brand/model.

Fixing...
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https://code.launchpad.net/~rcj/livecd-rootfs/trunk/+merge/355791
Your team Ubuntu Core Development Team is requested to review the proposed merge of lp:~rcj/livecd-rootfs/trunk into lp:livecd-rootfs.