minimize radioconfig file writes

src/mesh/NodeDB.h

@@ -43,7 +43,7 @@ class NodeDB
     void init();
 
     /// write to flash
-    void saveToDisk();
+    void saveToDisk(), saveChannelsToDisk();
 
     /** Reinit radio config if needed, because either:
      * a) sometimes a buggy android app might send us bogus settings or