lib/buildstats: handle tasks that never finished

If a task is aborted the buildstats file isn't complete, so calculate
when the build finished and use that as a end time.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 23ebaec4)
Signed-off-by: Steve Sakoman <steve@sakoman.com>