Changed reaction for uart DMA buffer overruns: In the previous version a DMA was not restarted in case the read pointer is in the chunk which should be filled next. In normal operation this should never happen, in special cases, like debugging, it may occure. In case of an overrun the data may be already outdated and can be discarded. Because of this the new handling of a buffer overflow is to set the read pointer to the start of the chunk => the read function will start with the latest received data. Not yet processed data will be discarded.