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
Couldn't you add something to the makefile to check if the user exported something like `MAX_ROUTING_ENTRIES` and then set `RPL_MAX_ROUTING_ENTRIES` accordingly?