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
A little thing I got pointed to: would it make sense to wait until the `USART_SR_TC` is set so that any ongoing transmission can be completed before powering off?