[Merge] ~toabctl/livecd-rootfs:bionic-repostamp-backports into livecd-rootfs:ubuntu/bionic

[Thomas Bechtold]

I did another testrun (without theses changes and repo-snapshot-stamp enabled). That passed for Bionic so this change is not needed. See https://launchpad.net/~cloudware/+livefs/ubuntu/bionic/cpc-development/+build/300730 for test results.
-- 
https://code.launchpad.net/~toabctl/livecd-rootfs/+git/livecd-rootfs-1/+merge/409292
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~toabctl/livecd-rootfs:bionic-repostamp-backports into livecd-rootfs:ubuntu/bionic.