rlink throughput on basys3/nexys4 limited by serial port stack round trip times

[wfjm]

The communication to basys3 and nexys4 boards is handled via the serial port driver. The combination of the FTDI USB UART and the Linux serial port stack gives a round trip time of 2 ms. On nexys3 and nexys2 boards and communication via the Cypress FX2 one gets a round trip time of 0.125 ms.

In many cases rlink throughput is limited by round trip time. It seems possible to improve it on basys3 and nexys4 boards with a libusb based custom driver.

originally filed as issue V0.64-4 on 2015-03-01