click chroot: use a PPA rather than -security/-updates

[Colin Watson]

Various people are concerned about ABI incompatibilities (perhaps just new binaries not working on old devices even if not vice versa) being introduced by -security/-updates. Security updates are not particularly important for "click chroot" by itself, but we do need to have an updates channel so that we don't paint ourselves into a corner for later. Let's define a PPA namespace which we use by default in "click chroot" rather than -security/-updates.