sys:net:routing:rpl allow setting the number of routing entries on comile time

This PR allows to configure the maximum used entries for the routing table during compile time, e.g.:
`make RPL_MAX_ROUTING_ENTRIES=99`
If no maximum number is provided `128` is assumed as default.
This PR closes #2082
Mostly, but we need to distinguish between root and non-root nodes for `(RPL_DEFAULT_MOP == RPL_NON_STORING_MODE)`.