Update code to match the updated spec about FixedAtBuild PCD.

Reviewed-by: Jiang Liu <jiang.a.liu@intel.com>
Signed-off-by: Yingke Liu <yingke.d.liu@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2-buildtools/code/trunk/BaseTools@2630 c2973edb-eda0-4c78-bc6a-9341b269661f