BaseTools: Enable --genfds-multi-thread to default build

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1302

This patch enable --genfds-multi-thread to default build.
This patch keep --genfds-multi-thread build option for
compatibility and also add a new build option to disable
genfds-multi-thread as --no-genfds-multi-thread.

Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/GenFds/GenFdsGlobalVariable.py

@@ -69,7 +69,7 @@ class GenFdsGlobalVariable:
     SecCmdList = []
     CopyList   = []
     ModuleFile = ''
-    EnableGenfdsMultiThread = False
+    EnableGenfdsMultiThread = True
 
     #
     # The list whose element are flags to indicate if large FFS or SECTION files exist in FV.