ftrace/selftests: Add softlink to latest log directory

When I'm debugging something with the ftrace selftests and need to look at
the logs, it becomes tedious that I need to do the following:

 ls -ltr logs
 [ copy the last directory ]
 ls logs/<paste-last-dir>

to see where the logs are.

Instead, do the common practice of having a "latest" softlink to the last
run selftest. This way after running the selftest I only need to do:

 ls logs/latest/

and it will always give me the directory of the last run selftest logs!

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>