buildman: Start the clock when the build starts

The Kconfig and maintainer processing can take a while, sometimes 5
seconds or more. This skews the timing printed by buildmand when the build
completes. Start the clock when the threads start to avoid this problem.

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