scripts/contrib/build-perf-test.sh: change the global results format

scripts/contrib/build-perf-test.sh: change the global results format

When all builds have finished write the hostname, commit and times
on a single line in the global results file (useful for merging later
on files from multiple systems).
Also the final cleaning should be last after writing the results.

(From OE-Core rev: 582798f70bf350d2db6911eb8df333ada05f6484)

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>