commonlib/device_tree: Add an API to check if a DT is an overlay

Add dt_is_overlay() API to check whether the input devicetree is
actually an overlay DT. Payload will use this API when parsing an input
image which is a collection of base and overlay devicetree blobs.

BUG=b:394980221
TEST=Build firmware image for Rauru/Hylia and boot to OS. Ensure that
the API correctly identifies the base and overlay DTs.

Change-Id: I2fc54e3d9e63ebc993c8ce6a7d4a7224a9251497
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90028
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/commonlib/device_tree.c

@@ -2149,3 +2149,19 @@ int dt_apply_overlay(struct device_tree *tree, struct device_tree *overlay)
 
 	return 0;
 }
+
+bool dt_is_overlay(struct device_tree *tree)
+{
+	/* The actual overlaid nodes/props are in an __overlay__ child node. */
+	const char * const overlay_path[] = { "__overlay__", NULL };
+	struct device_tree_node *fragment;
+
+	if (!tree)
+		return false;
+
+	list_for_each(fragment, tree->root->children, list_node)
+		if (dt_find_node(fragment, overlay_path, NULL, NULL, 0))
+			return true;
+
+	return false;
+}

src/commonlib/include/commonlib/device_tree.h

@@ -234,4 +234,7 @@ int dt_apply_fixups(struct device_tree *tree);
  */
 struct device_tree_node *dt_init_reserved_memory_node(struct device_tree *tree);
 
+/* Check whether a devicetree is an overlay device tree */
+bool dt_is_overlay(struct device_tree *tree);
+
 #endif /* __COMMONLIB_DEVICE_TREE_H__ */