AP_NavEKF2: Allow immediate use of GPS in-flight for plane without compass

Planes operating without a compass must align the yaw to the GPS heading immediately after launch or takeoff and cannot wait for GPS accuracy checks.

libraries/AP_NavEKF2/AP_NavEKF2_VehicleStatus.cpp

@@ -24,6 +24,12 @@ extern const AP_HAL::HAL& hal;
 */
 bool NavEKF2_core::calcGpsGoodToAlign(void)
 {
+    if (inFlight && assume_zero_sideslip() && !use_compass()) {
+        // this is a special case where a plane has launched without magnetometer
+        // is now in the air and needs to align yaw to the GPS and start navigating as soon as possible
+        return true;
+    }
+
     // User defined multiplier to be applied to check thresholds
     float checkScaler = 0.01f*(float)frontend->_gpsCheckScaler;