[grubcfg] Fix config schema

src/modules/grubcfg/grubcfg.schema.yaml

@@ -4,12 +4,14 @@ $id: https://calamares.io/schemas/grubcfg
 additionalProperties: false
 type: object
 properties:
-    "overwrite": { type: boolean, default: false }
-    "defaults":
-        type: map
-        mapping:
-            "GRUB_TIMEOUT": { type: int, required: true }
-            "GRUB_DEFAULT": { type: string, required: true }
-            "GRUB_DISABLE_SUBMENU": { type: boolean, default: true }
-            "GRUB_TERMINAL_OUTPUT": { type: string, required: true }
-            "GRUB_DISABLE_RECOVERY": { type: boolean, default: true }
+    overwrite: { type: boolean, default: false }
+    defaults:
+        type: object
+        additionalProperties: true  # Other fields are acceptable
+        properties:
+            GRUB_TIMEOUT: { type: integer }
+            GRUB_DEFAULT: { type: string }
+            GRUB_DISABLE_SUBMENU: { type: boolean, default: true }
+            GRUB_TERMINAL_OUTPUT: { type: string }
+            GRUB_DISABLE_RECOVERY: { type: boolean, default: true }
+        required: [ GRUB_TIMEOUT, GRUB_DEFAULT, GRUB_TERMINAL_OUTPUT ]