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
I think we can omit the second part of this condition since we just expect the pin to be input (00) or output (01). Calling `gpio_read()` on a un-initialized pin will end up in undefined behavior in any case...