BaseTools: Update Python Makefile not to depend on PYTHON_FREEZER_PATH

BaseTools: Update Python Makefile not to depend on PYTHON_FREEZER_PATH

If PYTHON_FREEZER_PATH is not set, Python tools will not be freeze.

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>