stages/grub2.legacy: make schema stricter

The code requires that `id`, `product` and `kernel` are defined for
each entry. Reflect that in the schema. Require at least one entry.

stages/org.osbuild.grub2.legacy

@@ -131,9 +131,11 @@ SCHEMA = """
   "entries": {
     "description": "List of entries to add to the boot menu",
     "type": "array",
+    "minItems": 1,
     "items": {
       "type": "object",
       "additionalProperties": false,
+      "required": ["id", "product", "kernel"],
       "properties": {
         "default": {
           "type": "boolean",