Add VCP driver (work in progress)
STM32F30x - Ensure static constructors are called.
STM32F30x - Fix USB VCP initialisation.
NAZE32PRO - Update timer configuration.  Work in progress.
Update makefile for non-NAZE32PRO targets.  Enable VCP on
STM32F3DISCOVERY.
Update makefile to use target specific files.

This is an alternative to #ifdef's everywhere which is getting out of
hand.

Confirmed that the VCP is working on the STM32F3Discovery board and
ChebuzzF3 variant.