Setup firmware to use RIOT

[jeandudey]

In the first revisions of the firmware we used Contiki-NG as our OS. For maintainability decisions we now are going to use RIOT.

Here is a checklist of what needs to be done:

• [x] Remove the contiki-ng project
• [x] Set a RIOT submodule pointing to our fork.
• [x] Add a basic project structure.

[jeandudey]

Already done! :tada: