AP_GPS: Swift Binary Protocol SINGLE-POINT-POSITIONING Driver

This is the SBP driver for Piksi, Lodestar and other forthcoming Swift Navigation GPSes.
This driver currently implements three things:
- Implements a lightweight SBP protocol detection system
- Implements Piksi as a single-point-positioning GPS (same as ublox/others)
- Implements hardware logging of GPS health and baseline messages

Forthcoming iin future updates:
- Need to implement the RTK functionality