tracing: Change trace_boot to use synth_event interface

Have trace_boot_add_synth_event() use the synth_event interface.

Also, rename synth_event_run_cmd() to synth_event_run_command() now
that trace_boot's version is gone.

Link: http://lkml.kernel.org/r/94f1fa0e31846d0bddca916b8663404b20559e34.1580323897.git.zanussi@kernel.org

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>