-moved rc.mavlink to the boards optional rc additions (now it's called rc.board_mavlink) to handle board specific mavlink needs (mavlink over usb, ethernet, additional streams, etc.)
-mavlink module will be responsible to usb defaults, therefore less args are needed to be passed to mavlink module if one wants to use mavlink over usb.
-the way to check if connection is usb is by it's designated variable and not by config mode.
* drivers, magnetometer: added ak09916 driver to support the mag driver emulated by the Here2 gps
* boards: set yaw 270 rotation for external ak09916 driver and only start it for the PH21
* drivers: only start ak09916 when icm20948 is not available on a ph21 setup
Daniel Agar