tools/genboardscfg.py: Don't rely on subprocess.check_output

The function subprocess.check_output() only exists in Python 2.7 and
later (and there are long term supported distributions that ship with
2.6.x).  Replace this with a call to subprocess.Popen() and then checking
output via communicate()

Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>