mavlink C++ified

This PR contains the C++ified mavlink module written by @LorenzMeier and debugged by me.

It has already been flight tested on fixedwing and multicopter, however, I propose another test round including the latest changes.

Please report any regressions or features which were working previously.
Specific onboard features are not yet implemented.

Attention, some changes in the defconfig are needed for this, so to build, do:
```
make distclean
make archives
make
```