Need the ability to configure player 2 / 3 / 4

[joolswills]

I know recalbox has this on their fork, although their controller setup is different of course etc, but it would be useful for us to have the ability to configure another controller, and specify it as player 2 / 3 / 4 - that could then be passed to our controller generation and in the case of retroarch keyboard configs, or pifba it would configure the correct player etc.

In the meantime I will implement a mechanism to configure player 2 / 3 / 4 for keyboard for retroarch in the existing retropie-setup input configuration.