wic: decode output of subprocess.communicate

wic: decode output of subprocess.communicate

stdeout and stderr content returned by communicate API has different
types in Python 3(bytes) and Python 2(string). Decoding it to 'utf-8'
makes it unicode on both pythons.

Decoded stdout and stderr output to utf-8 to make the code
working under both Python 2 and Python 3.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>