tools/bootconfig: Fix enabling all events in bconf2ftrace.sh

A bootconfig containing ftrace.event.enable makes bconf2ftrace.sh try to
write to events/enable/enable. The per-event loop treats the global
enable key as an event named "enable" in a group with the same name.

Since events/enable is a file, this write fails. With --apply, the script
exits before it reaches the code that enables all events. The same thing
happens with ftrace.instance.NAME.event.enable.

Skip the global enable key in the per-event loop and let the existing
global handler write to events/enable.

Link:

Fixes: 1d8365a553a7 ("tools/bootconfig: Support per-group/all event enabling option")
Signed-off-by: Eduardo Mucelli Rezende Oliveira <edumucelli@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>