Rover: only report ahrs unhealthy after initialisation

APMrover2/GCS_Mavlink.pde

@@ -169,7 +169,7 @@ static NOINLINE void send_extended_status1(mavlink_channel_t chan)
         control_sensors_health &= ~MAV_SYS_STATUS_SENSOR_3D_ACCEL;
     }
 
-    if (!ahrs.healthy()) {
+    if (ahrs.initialised() && !ahrs.healthy()) {
         // AHRS subsystem is unhealthy
         control_sensors_health &= ~MAV_SYS_STATUS_AHRS;
     }