Add support for the second stm32f4 CAN interface.

This is copied from the stm32f2 code with some #if's added so that you can
select the MCU pins used for the CAN signals at compile time. Four configurations
are supported.

For example, the board config file can contain this:

  macros = { 'ELUA_BOARD_STM32F4_CAN_PIN_CONFIG_2' }
Merge pull request #38 from smartavionics/mb-stm32f4-can2

Add support for the second stm32f4 CAN interface.