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>