Plane: move init of DataFlash references into vehicle init

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

ArduPlane/Log.cpp

@@ -561,11 +561,6 @@ void Plane::Log_Write_Vehicle_Startup_Messages()
 // start a new log
 void Plane::start_logging() 
 {
-    DataFlash.set_mission(&mission);
-    DataFlash.setVehicle_Startup_Log_Writer(
-        FUNCTOR_BIND(&plane, &Plane::Log_Write_Vehicle_Startup_Messages, void)
-        );
-
     DataFlash.StartNewLog();
 }
 

ArduPlane/system.cpp

@@ -297,6 +297,12 @@ void Plane::startup_ground(void)
     // initialise mission library
     mission.init();
 
+    // initialise DataFlash library
+    DataFlash.set_mission(&mission);
+    DataFlash.setVehicle_Startup_Log_Writer(
+        FUNCTOR_BIND(&plane, &Plane::Log_Write_Vehicle_Startup_Messages, void)
+        );
+
     // reset last heartbeat time, so we don't trigger failsafe on slow
     // startup
     failsafe.last_heartbeat_ms = millis();