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


This is quite a large block of commented out code. Can you remove it? And can you please use this in the future:

```
Linux style for comments is the C89 "/* ... */" style.
Don't use C99-style "// ..." comments.

The preferred style for long (multi-line) comments is:

	/*
	 * This is the preferred style for multi-line
	 * comments in the Linux kernel source code.
	 * Please use it consistently.
	 *
	 * Description:  A column of asterisks on the left side,
	 * with beginning and ending almost-blank lines.
	 */
```

Note the * at the beginning of the line. It is way easier to spot across all editors and notably on github.