Single firmware for split keyboards

[leiserfg]

It would be nice if we could have only a single source (and binary) for both sides, by recognizing which side is left or right and which one is central or peripheral via a provided function from the user. I come from KMK and there the side is usually gathered by

reading the label of the fat32 partition (on devices that can provide one, like rp2040)
checking a couple of physically  bridged pins as marker of the right side.

and for the central, the presence of USB connection makes it act as such, or else it falls back to being a peripheral.

[HaoboGu]

It's not possible for wireless split, because the mac addresses are different.

I'm happy to have it for wired split through, if someone wants to try