use DMA for serial communication

[Veix123]

The NUCLEO board has a DMA controller that could be used to offload some CPU load to the dedicated controller. @oalaro already looked into this in becd7c7. Sending a single message via DMA works, but afterwards everything crashes. Since DMA functions are not implemented in mbed-os yet we have to fall back to the lower level HAL layer.

[oalaro]

Found a working example online. Seems to work fine with sending and receiving data over serial with DMA. Needs more work to understand how everything works in this example and how to port it over completely to our use case.

b4847e4