[ground_segment] improve readability of icons and add a few more
[telemetry] disabling telemetry made easier

- reorganize telemetry makefiles for FW
- protect some calls and provide default implementation of
  register_periodic
- improve generator with attribute unused keyword
[conf] fix telemetry for sim and test progs

test progs don't use periodic telemetry
[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
[icons] fix icons size to have 48x48px
[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...
update CHANGELOG
[ext] make mavlink a submodule