BaseTools/toolsetup: Do not set CONF_PATH when already set.

BaseTools/toolsetup: Do not set CONF_PATH when already set.

When CONF_PATH is already set, toolsetup.bat overwrites its value.
This is not the case on Linux platforms (BuildEnv) and
contra-productive when using the same Workspace across multiple
Operating Systems.
With this patch, a check is performed prior to setting the variable.
Furthermore, it will not be scanned for Conf directories in
PACKAGES_PATH directories to respect the user's choice.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>