IntelFsp2Pkg/GenCfgOpt.py: support FixedAtBuild PCD

IntelFsp2Pkg/GenCfgOpt.py: support FixedAtBuild PCD

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1210

FixedAtBuild PCD is suggested to be used instead
of FeatureFlag PCD so extend this tool to support.
Also skipped PCDs which commented out by '#'.

Test: Verified with FixedAtBuild PCD for including or
excluding lines in generated UPD header files successfully.

Cc: Jiewen Yao <Jiewen.yao@intel.com>
Cc: Gao Liming <liming.gao@intel.com>
Cc: Zhu Yonghong <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>