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
Actually I would omit the change to the Makefile and document that a developer can specify this as a command line parameter, e.g. in the `rpl_udp` README. Having this `ifndef` here also allows the application developer to overwrite this from an application header.