tier-0: Rename to minimal

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>

fedora-tier-0.yaml

@@ -1,8 +0,0 @@
-metadata:
-  name: fedora-boot-tier0
-  summary: Fedora Bootable Tier 0
-
-include:
-  - fedora-generic.yaml
-  - tier-0/manifest.yaml
-  - tier-0/kernel.yaml