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/AprioriSection.py

@@ -53,7 +53,7 @@ class AprioriSection (object):
     #   @retval string      Generated file name
     #
     def GenFfs (self, FvName, Dict = {}, IsMakefile = False):
-        Buffer = BytesIO('')
+        Buffer = BytesIO()
         if self.AprioriType == "PEI":
             AprioriFileGuid = PEI_APRIORI_GUID
         else: