Zephyr restructured its includes in v2.0 and removed compatibility shims
after two releases in commit 1342dadc365ee22199e51779185899ddf7478686.
Updates include paths in MicroPython accordingly to fix build errors in
the zephyr port.
These changes are compatible with Zephyr v2.0 and later.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.