[Merge] ~mwhudson/livecd-rootfs/+git/livecd-rootfs:README.parameters into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

> $MIRROR seems to be available to override https://ubuntu-archive-
> team.ubuntu.com/seeds/

MIRROR in the environment livecd-rootfs runs in can be used to override the archive mirror (so http://ftpmaster.internal/ubuntu/ or http://archive.ubuntu.com/ubuntu or http://ports.ubuntu.com/ubuntu-ports/) but it can't be set via launchpad-buildd for a build in Launchpad.

SEEDMIRROR cannot be overridden (which is probably something we should fix, it makes it very annoying to test seed changes, but one thing at a time)
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https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/496283
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