Append to PyIncDirs, used to define include directories specific to
MicroPython, instead of just overwriting it so project files importing this
file can define additional directories. And allow defining the target
directory for the executable instead of hardcoding it to the windows
directory. Main reason for this change is that it will allow building
mpy-cross with msvc.
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.
stijn