Sensor drivers

These are sensor driver improvements / new laser rangefinder kindly contributed by the APM dev community.
yep, but for now I'm trying to use a single fw for both FMUv2 and FMUv3. The strategy at the moment is to carefully craft the startup script to avoid doing anything nasty on FMUv2. The detection strategy is to look for the external MPU6000 first, and if we find that then start other external sensors. That means we are setting all the external CS lines high on startup on FMUv2, and briefly lowering the external MPU6000 CS pin, and relying on that not causing a problem on FMUv2.
We could instead have a different fw, or come up with some other method of detecting the board type. I'm certainly open to suggestions!