sanitycheck.c: Add power-on grace period

Introduce a delay after powering up the flight controller to prevent arming before flight/Modules/Attitude/attitude.c has completed it's warmup routine, so that gyro calibration will have a higher chance of obtaining sensible values.

flight/Libraries/sanitycheck.c

@@ -292,6 +292,10 @@ static int32_t check_stabilization_settings(int index, bool multirotor)
  */
 static int32_t check_safe_to_arm()
 {
+	// 8 Second grace period while gyrobias completes warmup
+	if (PIOS_Thread_Systime() < 8000) {
+		return SYSTEMALARMS_CONFIGERROR_UNSAFETOARM;
+	}
 	FlightStatusData flightStatus;
 	FlightStatusGet(&flightStatus);