ftrace: ftrace_dump_on_oops=[tracer]

Impact: add new (optional) debug boot option

In order to facilitate early boot trouble, allow one to specify a tracer
on the kernel boot line.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>