cc2530dk: Added a 'Serial over USB' platform process

This is based on a usb-test example by Philippe Retornaz. It has
been moved to platform and modified accordingly. With this in place:

- putchar() can work over USB. So we can use things like
  printf, slip output
- USB input can be redirected to slip or serial input

The example itself is no longer needed in the source tree

See Pull Request #18