resulttool/log: Add ability to dump ltp logs as well as ptest

Currently only ptest logs are accessible with the log command, this
adds support so the ltp logs can be extracted too.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 64a2121a)
Signed-off-by: Steve Sakoman <steve@sakoman.com>