tools build feature: Introduce feature_assign macro

tools build feature: Introduce feature_assign macro

The feature_assign macro generates feature value
assignment for name, like:

  $(call feature_assign,dwarf) == feature-dwarf=1

This will be used more in following patches.

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/1450893514-9158-4-git-send-email-jolsa@kernel.org
[ Rename it to feature_assign, the original shorter name was misleading, to say the least ;-) ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>