ArduCopter: move VehicleClass handling to AHRS frontend

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

ArduCopter/system.cpp

@@ -213,7 +213,7 @@ void Copter::startup_INS_ground()
 {
     // initialise ahrs (may push imu calibration into the mpu6000 if using that device).
     ahrs.init();
-    ahrs.set_vehicle_class(AHRS_VEHICLE_COPTER);
+    ahrs.set_vehicle_class(AP_AHRS::VehicleClass::COPTER);
 
     // Warm up and calibrate gyro offsets
     ins.init(scheduler.get_loop_rate_hz());