python: add $(PKG_INSTALL_DIR)/$(PYTHON_PKG_DIR) to PYTHONPATH

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python-pip: remove explicit PYTHONPATH set

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python-setuptools: remove explicit PYTHONPATH set

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Merge pull request #619 from commodo/python-cleanup

Python cleanup