raburton/rboot ? reverse-engineered prompt

Build me an open source bootloader for the ESP8266 that replaces the stock SDK loader and can switch between multiple firmware images at boot.

I want it to support a handful of ROMs by default, with the option to increase that, handle variable sized images, and let me choose the next boot image from a config sector in flash. It should also support a GPIO based boot option so holding a pin low can either boot a specific backup image or skip to the next one. If a config sector is missing or corrupted, create a sensible default so the device still boots.

Please make it easy to build with the ESP8266 cross compiler and the SDK, and include clear notes for flashing the bootloader and firmware images to the right addresses. If needed, look up current docs online for the toolchain or flashing tools. Keep the code portable enough that if assembler stubs are a problem there is a plain C fallback.