BaseTools/toolsetup.bat: Fix bug caused by 'CONF_PATH' not defined

BaseTools/toolsetup.bat: Fix bug caused by 'CONF_PATH' not defined

In batch script files, setting a variable in an 'if' block will only take
effect after the 'if' block.

This commit fixes the issue of using the variable 'CONF_PATH' right after
it is being set in an 'if' block.

Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>