tools build feature: Use value assignment form for FEATURE-DUMP file

tools build feature: Use value assignment form for FEATURE-DUMP file

Changing the contents of the FEATURE-DUMP file, so it looks like:

  feature-backtrace=1
  feature-dwarf=0
  feature-fortify-source=1
  feature-sync-compare-and-swap=0

This way it could get included in sub projects, so they won't be forced
to redo features detection.

Also now storing the complete set of features.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Wang Nan <wangnan0@huawei.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama <pi3orama@163.com>
Link: http://lkml.kernel.org/r/1450893514-9158-5-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>