oe-build-perf-test: rename log file and implement --log-file

oe-build-perf-test: rename log file and implement --log-file

Rename the (main) log file of the oe-build-perf-test script from
'output.log' to 'oe-build-perf-test.log'. Also, add a new command line
option --log-file which makes it possible to use an alternative log file
name/path, if needed.  Note that the file name/path is relative to the
output directory.

(From OE-Core rev: 4909fae1a6d1d068b33252088b41b8d82d1a836c)

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