perf config: Allow '_' in config file variable names

perf config: Allow '_' in config file variable names

For annotate I want to be able to have variables that are the same as
the ones representing feature toggles.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-7rhhf6m0a72p2wja4tgv1itg@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>