Add USB joystick support?

[simoninns]

It should be possible to add USB joystick support; most retro computers place the axis movement on the same pins as the USB mouse quadrature (X1, X2, Y1 and Y2) and use the mouse button inputs for the fire switches.

Firmware would need to be enhanced to support both mouse and gamepad HID and auto-switch depending on the connected USB device.