tools: binman: change shebang from python into python2

This tool does not work with Python 3. Change the shebang to make sure the
script is run by a Python 2 interpreter.
Signed-off-by: NJörg Krause <joerg.krause@embedded.rocks>

tools/binman/binman.py

-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright (c) 2016 Google, Inc
 # Written by Simon Glass <sjg@chromium.org>