AP_NavEKF: Prevent arming delays from failing GPS

Due to the way that gyro calibration is done, the EKF could be effectively not run for up to 30 seconds in extreme cases, making it possible that the GPS would be failed on arming and the copter put into a non-GPS mode.

the longer term solution is to update the gyro calibration so that it does not hold up other processing. the short tyermfix in thsi patch is to look for evidence of a 3D lock in the last received GPS message.

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -4006,11 +4006,12 @@ void NavEKF::readGpsData()
         // calculate a position offset which is applied to NE position and velocity wherever it is used throughout code to allow GPS position jumps to be accommodated gradually
         decayGpsOffset();
     }
-    // If too long since last fix time or we are not allowed to use GPS, we declare no GPS available for use
-    if ((imuSampleTime_ms - lastFixTime_ms < 1000) || _fusionModeGPS == 3) {
-        gpsNotAvailable = false;
-    } else {
+
+    // If no previous GPS lock or told not to use it, we declare the  GPS unavailable available for use
+    if ((_ahrs->get_gps().status() < AP_GPS::GPS_OK_FIX_3D) || _fusionModeGPS == 3) {
         gpsNotAvailable = true;
+    } else {
+        gpsNotAvailable = false;
     }
 }