DDC-3.5.1-a055f1c

Release contains several differences from ArduCopter-3.5.1 release. Main of them are:

1. EKF status inside hearbeat message (as system_status) was added (for more correct DDC Drone statuses)
2. NORTH RTK<->Ublox GPS fusion logic.
3. Parameter GPS_ALT_THR was added, which determine GPS status when autopilot should switch to barometer.
4. Modified frequences for AUX1-6 PWM (for LEDs)

We also attach parameters files which we use during our flights (still we do not give any guarantees that they are correct for your usage). One of them is for use with RTK GPS onboard and another with standard GPS.