feat(core): Optionally disable automatic endpoint fallback

[ReFil]

By default if only one endpoint is connected but the other is selected, the connected endpoint will be chosen as a fallback, for boards with physical endpoint selection slide switches this can lead to confusing behaviour where one is selected but the other is used.

This adds a kConfig option CONFIG_ZMK_ENDPOINT_DISABLE_FALLBACK that, when enabled, switches the board to a different endpoint selection system that will only return the preferred transport or anew transport ZMK_TRANSPORT_NONE that just swallows any HID data.

Tested with various combinations of USB and BLE and it seems to work as intended

[caksoylar]

Can we add a link to https://zmk.dev/docs/keymaps/behaviors/outputs pointing to this? Perhaps an additional paragraph at the end of the summary section.

[joelspadin]

This looks like mostly duplicated code with get_selected_instance(). We should pull the common logic out into a separate function that both things can call so we don't keep the two functions in sync.

[joelspadin]

I'm not a fan of default cases with no logic. If you omit the default case, then the compiler can verify that you have a case for every enum value. Then, if you add a new enum value later but forget to update this code, it will give a diagnostic.

I suggest having a case for ZMK_TRANSPORT_NONE which just returns ZMK_TRANSPORT_NONE, and then treating it as an error if none of the cases are hit. Maybe even split this into two completely separate implementations of the function like this?

#if IS_ENABLED(CONFIG_ZMK_ENDPOINT_DISABLE_FALLBACK)

static enum zmk_transport get_selected_transport(void) {
    switch (preferred_transport) {
        case ZMK_TRANSPORT_BLE: 
            return ...
        case ZMK_TRANSPORT_USB: 
            return ...
        case ZMK_TRANSPORT_NONE: 
            return ZMK_TRANSPORT_NONE;
    }

    LOG_ERR("Unknown transport %d", preferred_transport);
    return ZMK_TRANSPORT_NONE;
}

#else

// original implementation

#endif