I've rebased the master branch including a few improvements:
AP_Compass has been separated in frontend and backend which makes it easier to create drivers for new sensors
AP_Compass support multiple instances
- AK8963 compass (which is available from the MPU9250 IC) is now active by default and will be used together with the GPS + Compass module to improve the readings.
Feel free to compile master and try it out. Note that you should calibrate the compass before flying.