py: Rename MP_OBJ_NOT_SUPPORTED to MP_OBJ_NULL.

See issue #608 for justification.
diff --git a/py/objbool.c b/py/objbool.c
index e31e7a2..4fe7a19 100644
--- a/py/objbool.c
+++ b/py/objbool.c
@@ -76,7 +76,7 @@
     if (MP_BINARY_OP_OR <= op && op <= MP_BINARY_OP_NOT_EQUAL) {
         return mp_binary_op(op, MP_OBJ_NEW_SMALL_INT((machine_int_t)mp_obj_is_true(lhs_in)), rhs_in);
     }
-    return MP_OBJ_NOT_SUPPORTED;
+    return MP_OBJ_NULL; // op not supported
 }
 
 const mp_obj_type_t mp_type_bool = {