Update contrib/mkimage/debootstrap to add "sqeeze-lts" to "oldstable" as well

Note that the archive does _not_ have a symlink from oldstable-lts to squeeze-lts, hence the change from using "${suite}" to not using it in here and just using the string "squeeze-lts" directly regardless of whether we're building a "squeeze" image or an "oldstable" image.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6897 from tianon/moar-squeeze-lts

Update contrib/mkimage/debootstrap to add "sqeeze-lts" to "oldstable" as well