Initial GSM driver for Ruuvitracker C2.

This is ported from previous eLua drivers to current ChibiOS implementation.
Not all features are supported (yet).
Just basic functionality.
Usage of global variables and API are to be improved later.
Basic debug(...) function
Initial HTTP GET/POST support.

Requires that GSM to be started and accespoint name is set.
Define gsm_start() and do not hardcode APN.
Example gsm&http test code