Arnaldo reported that 'make DEBUG=1' does not work anymore.

The reason is that 'Makefile' only passes it through to
'Makefile.perf' via the environment, but 'Makefile.perf'
checks that it's a command line option:

    ifeq ("$(origin DEBUG)", "command line")
      PERF_DEBUG = $(DEBUG)
    endif

So pass it through properly, and also clean up DEBUG parameter
handling while at it and fix a couple of annoyances:

 - DEBUG=0 used to be interpreted as 'debugging on'. Turn it
   into 'debugging off' instead.

 - Same was the case for 'DEBUG=' - turn that into debug-off
   as well.

 - Pass in just a clean, sanitized 'DEBUG' value and get rid of
   the intermediate, unnecessary PERF_DEBUG variable.
Reported-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>