image: Return destination node for add_verify_data() method

It is useful to know where the verification data was written. Update the
API to return this.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/rsa/rsa-sign.c

@@ -703,5 +703,8 @@ err_get_pub_key:
 	if (info->engine_id)
 		rsa_engine_remove(e);
 
-	return ret;
+	if (ret)
+		return ret;
+
+	return node;
 }