perf tools: Add descriptions of missing Makefile arguments

There are some variable arguments can be specified on make invocation,
but some of them are missing descriptions so that user cannot be
informed easily. Fix it.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1329980894-4289-1-git-send-email-namhyung.kim@lge.com


Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>