What is the status on merging up to lwip 2.1.2? I mentioned on a github issue for NONOS_SDK that I got this repo working with esp-open-sdk https://github.com/d-a-v/esp82xx-nonos-linklayer. From what I can tell, your edits implant the Espressif stuff directly into the source instead of that one that maps old functions to new ones and then has a bunch of patches. The mapping way is a bit clunky in my opinion, although it does make it easier to pull upstream changes for lwip. But lwip doesn't seem like the most active project.
I would prefer to compile lwip in my project's source so I'm thinking your way would be better. But I also want to have upstream fixes/features from lwip2. Do you see this being difficult?