Make rpl default route lifetime infinite

In high density network with DIO suppression activated it may happens that a node will never sent a DIO once the DIO interval becomes high as it has heard DIO from many neighbors already. As the default route to the preferred parent has a lifetime reset by receiving DIO from the parent, it means that the default route can be destroyed after a while.
This PR add the possibility to create default route with infinite lifetime, thus securing the upstream route. The default value is to not enable it though.
(It should be noted that TinyOS uses infinite lifetime for upstream routes in RPL)