py: Rename MP_OBJ_NOT_SUPPORTED to MP_OBJ_NULL.

See issue #608 for justification.
diff --git a/py/objint_longlong.c b/py/objint_longlong.c
index a9aea39..1e0167b 100644
--- a/py/objint_longlong.c
+++ b/py/objint_longlong.c
@@ -64,7 +64,7 @@
         case MP_UNARY_OP_POSITIVE: return o_in;
         case MP_UNARY_OP_NEGATIVE: return mp_obj_new_int_from_ll(-o->val);
         case MP_UNARY_OP_INVERT: return mp_obj_new_int_from_ll(~o->val);
-        default: return MP_OBJ_NOT_SUPPORTED;
+        default: return MP_OBJ_NULL; // op not supported
     }
 }
 
@@ -77,7 +77,7 @@
     } else if (MP_OBJ_IS_TYPE(lhs_in, &mp_type_int)) {
         lhs_val = ((mp_obj_int_t*)lhs_in)->val;
     } else {
-        return MP_OBJ_NOT_SUPPORTED;
+        return MP_OBJ_NULL; // op not supported
     }
 
     if (MP_OBJ_IS_SMALL_INT(rhs_in)) {
@@ -135,7 +135,7 @@
             return MP_BOOL(lhs_val == rhs_val);
 
         default:
-            return MP_OBJ_NOT_SUPPORTED;
+            return MP_OBJ_NULL; // op not supported
     }
 }