BaseTools/BinToPcd: Encode string returned from ByteArray()

BaseTools/BinToPcd: Encode string returned from ByteArray()

https://bugzilla.tianocore.org/show_bug.cgi?id=1069

The ByteArray() method returns a string with the hex bytes of
a PCD value.  Make sure the string is always encoded as a string,
so it can be used to build a complete PCD statement string and be
written out to a file.  This change is required for Python 3.x
compatibility.

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>