buildcommands: Use regular str() types for input on Python2

Avoid using unicode() types on Python2 as it can lead to subtle
errors.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

scripts/buildcommands.py

@@ -453,7 +453,7 @@ def check_output(prog):
     if retcode:
         return ""
     try:
-        return output.decode()
+        return str(output.decode('utf8'))
     except UnicodeError:
         logging.debug("Exception on decode: %s" % (traceback.format_exc(),))
         return ""