tracing/boot: Add per-group/all events enablement

Add ftrace.event.<GROUP>.enable and ftrace.event.enable
boot-time tracing, which enables all events under
given GROUP and all events respectivly.

Link: https://lkml.kernel.org/r/162264438005.302580.12019174481201855444.stgit@devnote2

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