oeqa.buildperf: rename buildstats directories

oeqa.buildperf: rename buildstats directories

Change directory name from 'buildstats-<test_name>' to just
'buildstats'. However, this patch adds the possibility to label
buildstats directory name with a postfix which makes it possible to save
multiple buildstats per test, for example.

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