Mavlink as git submodule

Following the discussions in https://github.com/PX4/Firmware/pull/1070 and https://github.com/mavlink/mavlink/pull/212 this PR adds mavlink as a submodule.
The repo is https://github.com/mavlink/c_library which contains just the autogenerated header files.

By using the git submodule we can keep the Firmware and QGC in sync by using the same commit of the c_library.

**This can not be merged as long as the windows toolchain is not updated to fetch git submodules**