I ran into this same issue and was able to make a bootable image by merging in the latest upstream buildroot master branch into the seed-linux-buildroot fork’s latest.
I can’t really speak to the stability of doing this however. There were no merge conflicts, but I had a few segfaults after booting the reTerminal, when trying to run the qml and lvgl_demo demos that come in the default image, but I don’t know if that’s related to the experiment of bringing in latest upstream buildroot or due to something else.