meta: allow mounts for all stages

If a stage has not itself defined the `mounts` property, allow any
mounts. This is in preparation to support specialized mounts, such
as bind mounts or ostree deployment mounts to transparently work
with any stage.
NB: devices are not allowed so this will not be applicable for the
current filesystem mounts.

osbuild/meta.py

@@ -331,6 +331,10 @@ class ModuleInfo:
                 type_id: {"enum": [self.name]},
                 **opts,
             }
+            if "mounts" not in schema["properties"]:
+                schema["properties"]["mounts"] = {
+                    "type": "array"
+                }
             schema["required"] = [type_id]
         elif self.type in ("Device"):
             schema["additionalProperties"] = True