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 MAX_ROUTING_ENTRIES=99`
If no maximum number is provided `128` is assumed as default.
This PR closes #2082
whats the benefit to set a default value in the header?
Right now the size of the entry array is completely controlled by the build system and the user.

> Also, I'm in favour of still prefixing this with RPL_
prefeixed.