lib: json: add JSON_OBJ_DESCR_PRIM

There are already helper macros for declaring descriptor fields of
object and array type. Add one for primitive types as well.

The fact that the JSON test code defines one proves that it's useful,
so there should be one provided for other users.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
1 file changed