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

[Tobias Koch]

Thanks to @philroche for testing the following scenarios:

1. Mixed - snaps with base core and core18 in image:

Both core and core18 were pulled in and added to the manifest correctly.

2. Only snaps with base core in image:

core was pulled in and added to the manifest correctly.

3. Only snaps with base core18 in image:

core18 was pulled in and added to the manifest correctly.
-- 
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.