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 <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Cody P Schafer <dev@codyps.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jeremie Galarneau <jeremie.galarneau@efficios.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kirill Smelkov <kirr@nexedi.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/n/tip-e3xzpx9cqsmwnaguaxyw6r42@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>