tracing/ftrace: add the boot tracer

Add the boot/initcall tracer.

It's primary purpose is to be able to trace the initcalls.

It is intended to be used with scripts/bootgraph.pl after some small
improvements.

Note that it is not active after its init. To avoid tracing (and so
crashing) before the whole tracing engine init, you have to explicitly
call start_boot_trace() after do_pre_smp_initcalls() to enable it.

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