perf tools: Introduce opt_event_config nonterminal

perf tools: Introduce opt_event_config nonterminal

To remove duplicated code that differs only in using the matching
'/a,b,c/' part or NULL if no event configuration is done ('//' or no
pair of slashes at all).

Will be used by some new targets allowing the configuration of hardware
events, etc.

Lifted part of the 'opt_event_config' nonterminal from a patch by Wang
Nan.

Cc: Adrian Hunter <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Brendan Gregg <[email protected]>
Cc: Cody P Schafer <[email protected]>
Cc: He Kuang <[email protected]>
Cc: Jeremie Galarneau <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kirill Smelkov <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Zefan Li <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>