BaseTools: Update toolsetup.bat to set PYTHONPATH env to run python source

BaseTools: Update toolsetup.bat to set PYTHONPATH env to run python source

When python tool exe doesn't exist, toolsetup.bat will set up PYTHONPATH,
and set python tool dos script directory into system PATH.

Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Erik Bjorge <erik.c.bjorge@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>