BaseTools/BinToPcd: Fix Python 2.7.x compatibility issue

BaseTools/BinToPcd: Fix Python 2.7.x compatibility issue

https://bugzilla.tianocore.org/show_bug.cgi?id=1042

Convert Buffer to type bytearray before converting to a
string of hex byte values so the type of items in Buffer is
consistent for both Python 2.7.x and Python 3.x.

Cc: YanYan Sun <yanyan.sun@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>