patman: Do not hardcode python path

Patman requires python 2.7.4 to run but it doesn't
need to be placed in /usr/bin/python.
Use env to ensure that the interpreter used is
the first one on environment's $PATH on system
with several versions of Python installed.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>