[Merge] ~powersj/livecd-rootfs/+git/livecd-rootfs:fix-unbound-variable into livecd-rootfs:ubuntu/master

[Robert C Jennings]

Review: Approve

+1, we can't get to the code which sets a default stable/ubuntu-$version channel if channel is unset without this MP. Thanks Josh
-- 
https://code.launchpad.net/~powersj/livecd-rootfs/+git/livecd-rootfs/+merge/387329
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~powersj/livecd-rootfs/+git/livecd-rootfs:fix-unbound-variable into livecd-rootfs:ubuntu/master.