base: add .dev marker by default

We want to be able to distinguish between release and non-release
builds. To do this, we embed a `.dev` marker in the version string so
that the default developer workflow will use this. As mentioned in the
previous commit message, this will be overridden for release streams.

See coreos/fedora-coreos-tracker#212 for more
information about this.

fedora-coreos-base.yaml

@@ -22,7 +22,7 @@ initramfs-args:
 machineid-compat: false
 
 releasever: "30"
-automatic-version-prefix: "${releasever}.<date:%Y%m%d>"
+automatic-version-prefix: "${releasever}.<date:%Y%m%d>.dev"
 mutate-os-release: "${releasever}"
 
 # Be minimal