Add Numato Mimas V2 support for MicroPython on FPGA & Linux on LiteX development

[mithro]

As the Numato Mimas V2 is small, portable & cheap, we have be using it as the primary target for doing MicroPython on FPGA and Linux on LiteX development.

For this reason, despite the board not really being useful for HDMI2USB stuff it would be good to still support it in hdmi2usb-mode-switch.

The board uses a PIC for the programming / serial UART, so isn't really "mode-switch able" but it could still support the --flash-gateware / --flash-bios / --flash-firmware commands. As well, there are a couple of different firmwares for the PIC, the original Numato firmware which requires manual changing of the switch, Jim's upgraded Numato Mimas V2 firmware -- so it would be good to support upgrading that and knowing which version is installed.