[navstik] Added a new board

Navstik STM32F4 autopilot.
This has the basic support for all the sensors and outputs most autopilots have. Still having issues with the i2c at boot, but seems to be a problem with STM32F4 autopilots.

https://wiki.paparazziuav.org/wiki/NavStik
http://www.navstik.org
All the other boards also define something like this. It isn't useful indeed but because it was in the others also I thought it would be nice to keep it consistent.