[mavlink] Basic implementation (needs cleanup)
[mavlink] configurable device
[modules] mavlink: fix time
[modules] mavlink: fix waring comparing to NULL
[modules] mavlink: send position
[modules] mavlink: works via usb_serial
[modules] mavlink: add some more messages

and silence some warnings
[modules] mavlink: easily configure UDP
[conf] add a mavlink test airframe
Merge pull request #1028 from paparazzi/mavlink_module

Very simple mavlink module that sends some basic messages.

A lot of things (like mav type, sensors, status, etc...) are still fixed and hardcoded, but good enough for first tests I guess...