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/AutoGen/ValidCheckingInfoObject.py

@@ -41,7 +41,7 @@ class VAR_CHECK_PCD_VARIABLE_TAB_CONTAINER(object):
             os.mkdir(dest)
         BinFileName = "PcdVarCheck.bin"
         BinFilePath = os.path.join(dest, BinFileName)
-        Buffer = ''
+        Buffer = bytearray()
         index = 0
         for var_check_tab in self.var_check_info:
             index += 1