GRC: add support for '\b' characters in the (graphical) console output.

During the initialization of e.g. the B210 the FPGA loading progress is shown and then updated using backspace characters. The console in the GRC main window can not process these resulting in a pretty ugly output. This patch emulates the behavior of a normal console. It deletes one char from the buffer for each '\b' at the beginning of the input line.