BaseTools: Fix incorrect %EDK_TOOLS_PATH%

BaseTools: Fix incorrect %EDK_TOOLS_PATH%

For non-root folder, such as "X:\test", the EDK_TOOLS_PATH will
resolve to "X:\test\edk2\BaseTools". This is OK.

But if WORKSPACE is at a root folder, such as "X:\", the EDK_TOOLS_PATH
will look like "X:\\BaseTools". The *double backslash* can fail the command
like "del" and thus affect the %ERRORLEVEL% variable, which may break
subsequent build processing.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yunhua Feng <yunhuax.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>