IntelFsp2Pkg/GenCfgOpt: skip unnecessarily header/BSF recreating.

IntelFsp2Pkg/GenCfgOpt: skip unnecessarily header/BSF recreating.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2967

When no change in FSP UPD DSC files, GenCfgOpt.py should skip
recreating UPD header and BSF files.
This patch added a check to handle this case.

Cc: Maurice Ma <maurice.ma@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>