BaseTools: Replace StringIO.StringIO with io.BytesIO

Replace StringIO.StringIO with io.BytesIO to be compatible with python3.
This commit also removes "import StringIO" from those python scripts
that don't really use it.

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

BaseTools/Source/Python/GenFds/FvImageSection.py

@@ -16,7 +16,7 @@
 # Import Modules
 #
 import Section
-import StringIO
+from io import BytesIO
 from Ffs import Ffs
 import subprocess
 from GenFdsGlobalVariable import GenFdsGlobalVariable
@@ -98,7 +98,7 @@ class FvImageSection(FvImageSectionClassObject):
         # Generate Fv
         #
         if self.FvName is not None:
-            Buffer = StringIO.StringIO('')
+            Buffer = BytesIO('')
             Fv = GenFdsGlobalVariable.FdfParser.Profile.FvDict.get(self.FvName)
             if Fv is not None:
                 self.Fv = Fv