Feature: Single code for splited keyboards

[leiserfg]

Type of Feature Request

• [X] Enhancement to an existing rumcake feature
• [ ] New feature unrelated to existing rumcake features
• [ ] New driver for existing rumcake features
• [ ] Support for a new MCU

Description

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 fiscally 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.