BaseTools/Scripts: Add ConvertUni.py script

BaseTools/Scripts: Add ConvertUni.py script

This script uses python codecs to convert .uni string files between
the utf-16 and utf-8 formats.

The advantages of utf-8 data:
 * Generally smaller files
 * More commonly supported by editors
 * Not treated as binary data in patch files

The script was tested on MdePkg with both python 2.7 and python 3.4.
It was able to convert all MdePkg .uni files between utf-8 and utf-16
multiple times always producing the same files for each format.

v2:
 * Rename ConvertUtf16ToUtf8.py to ConvertUni.py
 * Also support utf-8 to utf-16 conversion (with --utf-16)

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19247 6f19259b-4bc3-4df7-8a09-765794883524