Sub: move init of DataFlash references into vehicle init

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

ArduSub/Log.cpp

@@ -472,8 +472,6 @@ void Sub::start_logging()
     if (g.log_bitmask != 0 && !in_log_download) {
         if (!ap.logging_started) {
             ap.logging_started = true;
-            DataFlash.set_mission(&mission);
-            DataFlash.setVehicle_Startup_Log_Writer(FUNCTOR_BIND(&sub, &Sub::Log_Write_Vehicle_Startup_Messages, void));
             DataFlash.StartNewLog();
         } else if (!DataFlash.logging_started()) {
             hal.console->println("Starting new log");

ArduSub/system.cpp

@@ -165,6 +165,10 @@ void Sub::init_ardupilot()
     // initialise mission library
     mission.init();
 
+    // initialise DataFlash library
+    DataFlash.set_mission(&mission);
+    DataFlash.setVehicle_Startup_Log_Writer(FUNCTOR_BIND(&sub, &Sub::Log_Write_Vehicle_Startup_Messages, void));
+
     startup_INS_ground();
 
     // we don't want writes to the serial port to cause us to pause