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.
