tracing/ftrace: make tracing suitable to run the boot tracer

The tracing engine have now to be init in early_initcall to set the
boot tracer. Only the debugfs settings will be initialized at
fs_initcall time.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>