Uses the serial debug module and currently only works with
hardware/ulx3s, probably only on my specific machine where the
USB serial port is mapped _just so_. But it does work. Very WIP
unclean code, but checkpointing because it can hex view and hexedit
correctly.
In practice the flow control is unusable on ULX3S dev boards because
the CTS line isn't hooked up (it's instead wired to JTAG_TDO, to enable
the USB<>UART chip to serve a dual purpose as a bitbanged JTAG programmer)
Still, support for flow control is nice, for the future. And the UART
itself also works regardless of flow control, which is of course nice.
David Anderson