STM32: add CAN support for stm32f3

This enables the CAN support for stm32f3.
The code is almost the same, it just moved to the stm32/common/ folder.
The f3 has only one CAN interface, which resulted in some conflicts, so it had to be separated.
It is tested on f3 hardware.
This file doesn't exist.  You should probably also add this can file into the f2 makefile.

(Or, eliminate all the f2 references, but I'd of course rather see F2 added than left out :)