BaseTools: Support Structure PCD value assignment in DEC/DSC

https://bugzilla.tianocore.org/show_bug.cgi?id=542
This is pure BaseTools enhancement to support PCD with one structure.
User can specify PCD value based on its structure field.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>

BaseTools/Source/Python/Common/GlobalData.py

@@ -86,6 +86,9 @@ BuildOptionPcd = []
 #
 MixedPcd = {}
 
+# Structure Pcd dict
+gStructurePcd = {}
+
 # Pcd name for the Pcd which used in the Conditional directives
 gConditionalPcds = []