BaseTools: Handle the bytes and str difference

Deal with bytes and str is different, remove the unicode(),
correct open file parameter.
Using utcfromtimestamp instead of fromtimestamp.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Source/Python/GenFds/FvImageSection.py

@@ -102,7 +102,7 @@ class FvImageSection(FvImageSectionClassObject):
         # Generate Fv
         #
         if self.FvName is not None:
-            Buffer = BytesIO('')
+            Buffer = BytesIO()
             Fv = GenFdsGlobalVariable.FdfParser.Profile.FvDict.get(self.FvName)
             if Fv is not None:
                 self.Fv = Fv