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, that's a harder one currently the indication if a node is root is done [after/on initialization](https://github.com/RIOT-OS/RIOT/blob/master/sys/net/routing/rpl/rpl_nonstoring/rpl_nonstoring.c#L188) of that node. 
This information is not set on compile time currently.
We could force the user  to decide on compile time if the node is root, e.g. 
`make RPL_NODE_IS_ROOT` or similar