Issue#1556. Battery module: wrong string comparison

1. battery.hpp - added local bool variable. Force to print warnings the
only once in order to warn user about wrong battery configuraion. And
does not bring a mess when the battery is turned off (gamepads, etc.)
2. dir_name is an object which takes a part in comparison. So converted to the string.

include/modules/battery.hpp

@@ -46,6 +46,7 @@ class Battery : public ALabel {
   int global_watch_fd_;
   std::mutex battery_list_mutex_;
   std::string old_status_;
+  bool warnFirstTime_{true};
 
   util::SleeperThread thread_;
   util::SleeperThread thread_battery_update_;