Rover: move VehicleClass handling to AHRS frontend

.... and renaming the enumeration while we're at it

Rover/system.cpp

@@ -261,7 +261,7 @@ void Rover::startup_INS_ground(void)
     ahrs.init();
     // say to EKF that rover only move by going forward
     ahrs.set_fly_forward(true);
-    ahrs.set_vehicle_class(AHRS_VEHICLE_GROUND);
+    ahrs.set_vehicle_class(AP_AHRS::VehicleClass::GROUND);
 
     ins.init(scheduler.get_loop_rate_hz());
     ahrs.reset();