Pull out a MergeWithConfig helper

Change-Id: I1dc55a06a8fb3c20b1376ebb6f579495482ea94a
diff --git a/aos/configuration.cc b/aos/configuration.cc
index 43e44f2..f379166 100644
--- a/aos/configuration.cc
+++ b/aos/configuration.cc
@@ -458,11 +458,16 @@
 }
 
 FlatbufferDetachedBuffer<Configuration> MergeWithConfig(
+    const Configuration *config, const Flatbuffer<Configuration> &addition) {
+  return MergeConfiguration(MergeFlatBuffers(config, &addition.message()));
+}
+
+FlatbufferDetachedBuffer<Configuration> MergeWithConfig(
     const Configuration *config, std::string_view json) {
   FlatbufferDetachedBuffer<Configuration> addition =
       JsonToFlatbuffer(json, Configuration::MiniReflectTypeTable());
 
-  return MergeConfiguration(MergeFlatBuffers(config, &addition.message()));
+  return MergeWithConfig(config, addition);
 }
 
 const Channel *GetChannel(const Configuration *config, std::string_view name,
diff --git a/aos/configuration.h b/aos/configuration.h
index ccc6246..a01a902 100644
--- a/aos/configuration.h
+++ b/aos/configuration.h
@@ -36,6 +36,8 @@
 // returns the modified config.
 FlatbufferDetachedBuffer<Configuration> MergeWithConfig(
     const Configuration *config, std::string_view json);
+FlatbufferDetachedBuffer<Configuration> MergeWithConfig(
+    const Configuration *config, const Flatbuffer<Configuration> &addition);
 
 // Returns the resolved location for a name, type, and application name. Returns
 // nullptr if none is found.