Magnetometer devices and madgwick filter support added to linorobot2

[hipp...@gmail.com]

I added magnetometer devices and madgwick filter support to linorobot2 at hippo5329/linorobot2_hardware and hippo5329/linorobot2. Pull requests pr-mag are sent to upstream linorobot. Usage of magnetometer is updated in my [wiki](https://github.com/hippo5329/linorobot2_hardware/wiki). Testing and comments are welcome.

Cheers,

Thomas Chou