buildman: Clarify the use of -V

This option outputs to the log file, not to the terminal. Clarify that in
the help, and add a mention of it in the README.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>