[ardrone2] warn if bat_voltage_ardrone2 module is not loaded

sw/airborne/boards/ardrone/board.c

@@ -28,6 +28,12 @@
 #include <stdlib.h>
 #include "mcu.h"
 
+/* check if the electric_ardrone2 module is loaded */
+#include "generated/modules.h"
+#ifndef BAT_VOLTAGE_ARDRONE2_PERIODIC_FREQ
+#warning No battery voltage measurement available! Please add <load name="electric_ardrone2.xml"/> to your modules.
+#endif
+
 void board_init(void)
 {
   // First we try to kill the program.elf and its respawner if it is running