Merge pull request #2034 from ARMmbed/ws_aro_lifetime_fix

Fixxed ARO lifetime /60 +1 . now resolution is OK.

Author: Juha Heiskanen

source/RPL/rpl_downward.c

@@ -590,7 +590,7 @@ void rpl_instance_send_address_registration(protocol_interface_info_entry_t *int
 
     aro.status = ARO_SUCCESS;
     aro.present = true;
-    aro.lifetime = addr->valid_lifetime;
+    aro.lifetime = (addr->valid_lifetime / 60) + 1;
     memcpy(aro.eui64, interface->mac, 8);
 
     // go through neighbour list, and send to all assigned parents.