genrandconfig: use subprocess.check_output instead of Popen

Popen is more complicated and more difficult to understand.

check_output raises an exception if the exit code is non-zero, but
that's probably what we want if ldd can't be executed.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>