IntelFsp2Pkg/GenCfgOpt.py: Incremental build with UPD in sub DSC.

IntelFsp2Pkg/GenCfgOpt.py: Incremental build with UPD in sub DSC.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3107

Current script only compares main DSC and output file datetime
to determine if re-generation required or not.
When UPD defined in sub DSC and was modified current script cannot
detect and will not re-generate output files which caused incremental
build issue.

Since UPD can be defined in any sub DSC the script has been updated
to compare all DSC datetime with output files to determine re-generation
is needed or not.

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: Yuwei Chen <yuwei.chen@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Tested-by: Yuwei Chen <yuwei.chen@intel.com>