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