scripts: python3: decode subprocess output

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

(From OE-Core rev: ae4d36375683b6cfd48af25bfca70b2e77f7ade5)

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

scripts/combo-layer

@@ -198,7 +198,7 @@ def runcmd(cmd,destdir=None,printerr=True,out=None,env=None):
         raise e
 
     err.seek(0)
-    output = err.read()
+    output = err.read().decode('utf-8')
     logger.debug("output: %s" % output.replace(chr(0), '\\0'))
     return output