perf config: Add a function to set one variable in .perfconfig

To allow for setting a variable from some other tool, like with the
"wallclock" patchset needs to allow the user to opt-in to having
that key in the sort order for 'perf report'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/lkml/Z4akewi7UPXpagce@x1


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>