Ashtech GNSS receiver driver

Added driver for Ashtech GNSS receiver with 20Hz position update rate.

Ashtech driver will increase fix_type (visible in sdlog or QGC) to 4, 5 or 6 depending on RTCM/RTK differential status.

Tested on Ashtech MB100 board. 

The following logfile was recorded during flight without RTK diff corrections, only RTCM differential (SBAS) was used, so eph was about 0.5.
https://www.dropbox.com/s/mj5pssn6isxvb25/15_21_15.bin?dl=0

The following logfile was recorded during flight with RTK diff corrections and eph was about 0.02
https://www.dropbox.com/s/cj4m9sap6bl94l3/09_43_12.bin?dl=0


I removed VTG message completely, it's not being used now anyways.