toaster.bbclass: improve how we gather buildstats for Toaster
Clean up the code which gathers buildstats for Toaster,
and modify the field names so that the correct parts of the
buildstats files are used to derive the CPU usage values.
Also derive elapsed time for the build here, rather than in
Toaster, as we have ready access to the data in the correct
format.
[YOCTO #8842]
Signed-off-by: Elliot Smith <elliot.smith@intel.com> Signed-off-by: Michael Wood <michael.g.wood@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
