Plane: fixed mavlink levelling

this was broken by a recent commit (after 2.72 was released)

ArduPlane/GCS_Mavlink.pde

@@ -1136,7 +1136,7 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
         case MAV_CMD_PREFLIGHT_CALIBRATION:
             if (packet.param1 == 1 ||
                 packet.param2 == 1) {
-                startup_INS_ground();
+                startup_INS_ground(true);
             } else if (packet.param3 == 1) {
                 init_barometer();
                 if (airspeed.enabled()) {

ArduPlane/setup.pde

@@ -376,7 +376,7 @@ setup_erase(uint8_t argc, const Menu::arg *argv)
 static int8_t
 setup_level(uint8_t argc, const Menu::arg *argv)
 {
-    startup_INS_ground();
+    startup_INS_ground(true);
     return 0;
 }
 

ArduPlane/system.pde

@@ -281,7 +281,7 @@ static void startup_ground(void)
     //INS ground start
     //------------------------
     //
-    startup_INS_ground();
+    startup_INS_ground(false);
 
     // read the radio to set trims
     // ---------------------------
@@ -427,7 +427,7 @@ static void check_short_failsafe()
 }
 
 
-static void startup_INS_ground(void)
+static void startup_INS_ground(bool do_accel_init)
 {
 #if HIL_MODE != HIL_MODE_DISABLED
     while (!barometer.healthy) {
@@ -453,6 +453,10 @@ static void startup_INS_ground(void)
     ins.init(AP_InertialSensor::COLD_START, 
              ins_sample_rate,
              flash_leds);
+    if (do_accel_init) {
+        ins.init_accel(flash_leds);
+        ahrs.set_trim(Vector3f(0, 0, 0));
+    }
     ahrs.reset();
 
     // read Baro pressure at ground