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/cleanup-workdir

@@ -45,7 +45,7 @@ def run_command(cmd):
     if pipe.returncode != 0:
         print("Execute command '%s' failed." % cmd)
         sys.exit(1)
-    return output
+    return output.decode('utf-8')
 
 def get_cur_arch_dirs(workdir, arch_dirs):
     pattern = workdir + '/(.*?)/'