python-setuptools: Add missing $(TARGET_DIR) to PYTHONPATH

The $(TARGET_DIR) variable is required when building
python-setuptools for the target otherwise the build system detects
the host installation which leads to permission error problems
like these:

Setuptools installation detected at /usr/lib64/python2.7/site-packages
Renaming /usr/lib64/python2.7/site-packages/setuptools-0.9.8-py2.7.egg-info to
/usr/lib64/python2.7/site-packages/setuptools-0.9.8-py2.7.egg-info.OLD.1377005697.88
OSError: [Errno 13] Permission denied

Moreover, remove the PYTHONPATH variable for host variant since it's
not needed.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>