buildman: Use env to pick the python from $PATH

python used in buildman doesn't need to be placed in
/usr/bin/python, So use env to ensure that the interpreter
will pick the python from environment.

Usefull with several versions of python's installed on system.
Signed-off-by: NJagannadha Sutradharudu Teki <jaganna@xilinx.com>
Acked-by: NSimon Glass <sjg@chromium.org>

tools/buildman/buildman.py

-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright (c) 2012 The Chromium OS Authors.
 #