Separate FS write from serializeConfig

Break the actual JSON assembly apart from the file writing code.  This
permits calling it in other contexts, allowing us to pull the live
config data even if the filesystem is out of date.

wled00/improv.cpp

@@ -272,5 +272,5 @@ void parseWiFiCommand(char* rpcData) {
   improvActive = 2;
 
   forceReconnect = true;
-  serializeConfig();
+  serializeConfigToFS();
 }