Instead of separate rusage and child rusage values, only store their sum
value in buildstats. This is a big reduction in data footprint without
really losing any interesting data.
Also, utilize OrderedDict to order data more logically.
[YOCTO #10582]
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>