This refactors the build system to be more in line with the average
ESP-IDF project.

This means that:
  1. `idf.py` from the main directory now works
  2. Adding components, running menuconfig, etc. should now be trivial.
  3. We don't have to edit files in micropython/ports/esp32/....

Hopefully this is the first step to yeeting the micropython checkout
from source and just depending on a submodule instead.