ftrace: Rename set_bootup_ftrace into set_cmdline_ftrace

set_cmdline_ftrace is a better match against what does this function:
apply a tracer name from the kernel command line.

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