added pip to make install

As suggested, I added pip to make install.

However, I'm not sure whether this is how we want this. Packages now go to /usr/lib/micropython instead of /.micropython/lib. I'm not fully sure what's going on with the virtualenvs in this script.
Please review thoroughly and bounce in case.

Somehow I also managed to get some commits into this from an earlier experiment that shouldn't be there at all.
Thanks for your patience, I still have a lot to learn.
But which? TMPDIR is a standard POSIX environment var pointing to a temporary dir (which should exist). For people who happen not to have that, it just uses /tmp which is de facto standard for tmp dir. If you don't have either, what kind of OS do you use?

Then, "pip-micropy-venv" inside $TMPDIR should be created by virtualenv.

So, I don't oppose reinsuring and doing mkdir -p, I just would like to know circumstance under which it fails for you.