u_boot_pylib: Ensure subprocess is closed down

It isn't clear why we need to have two different paths for closing down
the pipe. Unify them and use the Python to avoid this warning:

  subprocess.py:1127: ResourceWarning: subprocess 83531 is still running

Note that this code appears to originally have come from [1] and was
committed into the ChromeOS chromiumos/platform/crosutils repo in the
bin/cros_image_to_target.py file. The addition of the extra code path
came later, so that is chosen for the fixes tag.

[1] https://codereview.chromium.org/3391008



Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: a10fd93c patman: Make command methods return a CommandResult