The CC2531 USB stick now identifies itself as a
'Texas Instruments CC2531 USB Dongle' and uses a
TI-assigmed VID:PID. The VID:PID is now configurable
in contiki- or project-conf.h
Change to have a real usb VID/PID and better fit the capabilities
of the CC2531 hardware (enpoint size, location).
Compile only if the cdc-acm class is requested.
See Pull Request #18