Rover: move init of DataFlash references into vehicle init

It is possible to start a log before the existing codepath is crossed.

APMrover2/Log.cpp

@@ -517,10 +517,6 @@ void Rover::Log_Write_Vehicle_Startup_Messages()
 void Rover::start_logging()
 {
     in_mavlink_delay = true;
-    DataFlash.set_mission(&mission);
-    DataFlash.setVehicle_Startup_Log_Writer(
-        FUNCTOR_BIND(&rover, &Rover::Log_Write_Vehicle_Startup_Messages, void)
-        );
     DataFlash.StartNewLog();
     in_mavlink_delay = false;
 }

APMrover2/system.cpp

@@ -249,6 +249,12 @@ void Rover::startup_ground(void)
     // initialise mission library
     mission.init();
 
+    // initialise DataFlash library
+    DataFlash.set_mission(&mission);
+    DataFlash.setVehicle_Startup_Log_Writer(
+        FUNCTOR_BIND(&rover, &Rover::Log_Write_Vehicle_Startup_Messages, void)
+        );
+
     // we don't want writes to the serial port to cause us to pause
     // so set serial ports non-blocking once we are ready to drive
     serial_manager.set_blocking_writes_all(false);