We don't pre-generate a FAT partition anymore.

Turns out there's a bug in ESP-IDF that causes FPU registers to get
correpted on context switches:

https://github.com/espressif/esp-idf/issues/11690

This applies this patch in nix, and reminds others to apply this patch
in the README. With this patch applied, graphical glitches in the
firmware seem gone.