buildman: Show a summary of the build result

buildman: Show a summary of the build result

When buildman finishes it leaves the last summary line visible, which
shows the number of successful builds, builds with warnings and builds
with errors.

It is useful also to see how many builds were done in total along with
the time taken. Show these on a separate line before buildman finishes.

Signed-off-by: Simon Glass <sjg@chromium.org>