Initial import of MSB-IoT board

Pull request contains basic support for the MSB-IoT board used at CST group at FU Berlin. It also adds support for more GPIO pins and one more UART to the corresponding stm32f4 drivers.

It is already based on the proposed updated low-level driver interfaces and hence depends on:
* #1481
* #1484
Yes, that was my first thought as well and it's probably the best way to handle it. I just want to hear Hauke's opinion because this would mean that the other UART_x_ISR functions should also be handled this way to keep it consistent.