tools lib traceevent: Make plugin options either string or boolean

tools lib traceevent: Make plugin options either string or boolean

When a plugin option is defined, by default it is a boolean (true or false).

If the option is something else, then it needs to set its "value" field to
a default string other than NULL (can be just "").

If the value is not set then the option is considered boolean, and the
updating of the option value will be handled accordingly.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20150324135923.308372986@goodmis.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>