BaseTools: Convert ".\\" to "" in FilePath

Convert ".\\" to "", because it doesn't work with WINDOWS_EXTENSION_PATH.
WINDOWS_EXTENSION_PATH can support the file path larger than 260 length.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Yingke Liu <yingke.d.liu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17686 6f19259b-4bc3-4df7-8a09-765794883524