[Merge] ~tobijk/livecd-rootfs/+git/livecd-rootfs:feature/implicit-core-seeding into livecd-rootfs:ubuntu/master

[Tobias Koch]

I made it so it would work incrementally, because it is difficult to decide implicitly if we are done with an image or if it is going to be remounted and more snaps added later. The last remaining issue here is that config/snapd-explicit-install-stamp is global, this would have to do into the image itself.
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https://code.launchpad.net/~tobijk/livecd-rootfs/+git/livecd-rootfs/+merge/365878
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