Addressing sketch size limitations

[schlimmchen]

The ESP32 (non-S3) build of OpenDTU is currently at 85% usage of the maximum permissible sketch size, limited by the size of the respective flash partition(s). This still leaves some room for updates, improvements, and additions. However, efforts to save flash were already made in the past in this project, suggesting that @tbnobody is well aware of the limitation.

The downstream project OpenDTU-OnBattery already is hitting the limit. More features require more code, currently leading to the inability to merge the recent upstream release without breaking the ESP32 builds using the 4MB flash partition scheme. We have been discussing this issue in helgeerbe#935 and I made an announcement in helgeerbe#1025 that going forward, only devices with 8 MB of flash would be supported by OpenDTU-OnBattery.

My latest suggestion is to keep support for devices with 4 MB of flash, but to remove OTA upgrade capabilities for those devices, effectively doubling the sketch size, as we would when transitioning to an 8 MB flash partition scheme. User with these devices can keep their devices and still update them, but need physical access and a USB connection to update the device.

The idea to introduce a minimal firmware (similar approach to Tasmota minimal firmware) is also still making the rounds. My first rough estimate is that a partition holding such a minimal firmware would need around 875k of flash. I argue against this approach, as it is high in maintenance.

@tbnobody Have you thought about addressing this limitation, even though the issue is not as pressing for OpenDTU as it is for OpenDTU-OnBattery? If so, do you mind discussing your ideas, so we can attempt to keep OpenDTU and OpenDTU-OnBattery compatible?