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

[Tobias Koch]

I have made the additions to make

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

work in commit 5afb4fba. Because this is a bit tricky, I also made a test script for it

https://pastebin.canonical.com/p/VGxJfjBGmF/

and the test log is here:

https://pastebin.canonical.com/p/c32QbyGzxC/
-- 
https://code.launchpad.net/~tobijk/livecd-rootfs/+git/livecd-rootfs/+merge/365878
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