BaseTools: Create and use a shared value for 'MSFT' from DataType

I see lots of 'MSFT' throughout code and this can reduce them.

Cc: Bob Feng <Bob.c.Feng@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>

BaseTools/Source/Python/GenFds/GenFdsGlobalVariable.py

@@ -66,8 +66,8 @@ class GenFdsGlobalVariable:
     FixedLoadAddress = False
     PlatformName = ''
 
-    BuildRuleFamily = "MSFT"
-    ToolChainFamily = "MSFT"
+    BuildRuleFamily = DataType.TAB_COMPILER_MSFT
+    ToolChainFamily = DataType.TAB_COMPILER_MSFT
     __BuildRuleDatabase = None
     GuidToolDefinition = {}
     FfsCmdDict = {}