scripts/buildstats-diff: implement --multi option

scripts/buildstats-diff: implement --multi option

Makes it possible to average over multiple buildstats. If --multi is
specified (and the given path is a directory) the script will read all
buildstats from the given directory and use averaged values calculated
from them.

All of the buildstats must be from a "similar" build, meaning that no
differences in package versions or tasks are allowed. Otherwise, the
script will exit with an error.

(From OE-Core rev: 315f44ba39e9b13facacd0fd3796fa87329d9d69)

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>