The "tiers" nomenclature ended up being unhelpful since
we introduced "tier-x" which is between tier-0 and tier-1.
We also never exposed the tier naming outside of our source
code. In preparation for doing so, rename to tier-0 to
"minimal" which is a bit more descriptive.
Renaming the other images will follow.
Signed-off-by: Colin Walters <walters@verbum.org>
09c902e1fc