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 would vote for taking the buttons out here. The rationale for having the LEDs controlled directly via macros is to have the fastest possible access to them, so they (an their pins) can be used for debugging timing issues. The buttons however do not have these timing requirements, so it is completely fine to just use them via the GPIO driver interface.