Several fixes were needed to allow following builds:
  $ make tools/tmon
  $ make -C <kernelsrc> tools/perf
  $ make -C <kernelsrc>/tools perf

 - some of the tools (perf) use same make variables as in
   kernel build, unsetting srctree and objtree
 - using original $(O) for O variable
 - perf build does not follow the descend function setup
   invoking it via it's own make rule

I tried the rest of the tools/Makefile targets and they
seem to work now.
Reported-by: NBrian Norris <computersforpeace@gmail.com>
Signed-off-by: NJiri Olsa <jolsa@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: linux-kbuild@vger.kernel.org
Link: http://lkml.kernel.org/r/1429389280-18720-3-git-send-email-jolsa@kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>