scripts/buildstats-diff: introduce --diff-attr

scripts/buildstats-diff: introduce --diff-attr

A new command line option for choosing which "attribute" of the
buildstats to compare. At first, the already supported 'cputime' is the
only available option. But, refactoring done in this patch should make
it easy to add new attribute types.

(From OE-Core rev: 0782825138731b3f1e6a8e05d723c1d5cd60c90c)

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>