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

[Tobias Koch]

@mwhudson According to 

https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/1820840/comments/3

"livecd-rootfs to detect when *all* seeded snaps specify a base:, and if so, seed snapd instead of core"

In the current state, in an image with only core18-based snaps preseeded, neither core nor snapd would be pulled in, only core18. I tried to manufacture a way to satisfy the above requirement only to realize that snapd has base 'core' and thus nothing is gained from seeding snapd *instead* of core.

But my understanding is that either snap is needed, so possibly this MP needs to be augmented to pull in either core or snapd (which would pull in core) when core18 is seeded.

This needs clarification with @vorlon.
-- 
https://code.launchpad.net/~tobijk/livecd-rootfs/+git/livecd-rootfs/+merge/365878
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~tobijk/livecd-rootfs/+git/livecd-rootfs:feature/implicit-core-seeding into livecd-rootfs:ubuntu/master.