tracing: Rename set_ftrace to set_bootup_ftrace

Do this rename because set_ftrace is too much generic and not enough
self-explainable as a name.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>