Open vSwitch needs to run python on build machine to make build-time
required files.  python-six library is only required by the openvswitch
python library on target machine, not a build dependency.

We override host PYTHONPATH by overriding it in MAKE_VARS.  This way we
can remove 0003-override-pythonpath-via-make-vars.patch

This also fixes shebang wrongly pointing to python on host

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>