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
Hmm, I would like to keep the possibility to set the maximum routing entries on compile time without need to change code.
But if I'm alone with this I just remove it, I'm not stick to this change.