tools/mpp: support multiple image definitions

Support more than one image definition by introducing a `id` attribute
which can be used to specify how the variable for the definition is
called.

test/data/manifests/fedora-ostree-image.mpp.json

@@ -1,6 +1,7 @@
 {
   "version": "2",
   "mpp-define-image": {
+    "id": "image",
     "size": "10737418240",
     "table": {
       "uuid": "D209C89E-EA5E-4FBD-B161-B461CCE297E0",

tools/osbuild-mpp

@@ -691,7 +691,8 @@ class ManifestFile:
 
         del self.root["mpp-define-image"]
 
-        self.vars["image"] = Image.from_dict(desc)
+        name = desc.get("id", "image")
+        self.vars[name] = Image.from_dict(desc)
 
 
 class ManifestFileV1(ManifestFile):