util: Look for python2 binary instead of python

Make the requirement of python2 explicit in scripts that are incompatible
with python3.

Change-Id: I77f150bdb3aab316fc3c3a21b911db397fa0106f
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/13286
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/dtd_parser/dtd_parser.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 # dtd_parser.py - DTD structure parser
 #
 # Copyright (C) 2012 The ChromiumOS Authors.  All rights reserved.