trace: fix default boot up tracer

Peter Zijlstra started the functionality to start up a default
tracing at bootup. This patch finishes the work.

Now if you add 'ftrace=<tracer>' to the command line, when that tracer
is registered on bootup, that tracer is selected and starts tracing.

Note, all selftests for tracers that are registered after this tracer
is disabled. This prevents the selftests from disturbing the running
tracer, or the running tracer from disturbing the selftest.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>