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
How about:
```
#ifndef MAX_ROUTING_ENTRIES
#    define MAX_ROUTING_ENTRIES    (16) /* or whatever seems to be sensible to you as a default value */
#endif
```
?

Also, I'm in favour of still prefixing this with `RPL_`