sys:net:routing:rpl changed `rpl_routing_table` to be dynamically allocated

The former statically allocated `rpl_routing_table[]` "wasted" a lot of memory when the `RPL_MAX_ROUTING_ENTRIES` haven't been completely used.

This PR replaces the static allocated entries with dynamically handled ones, starting with a "small" number of entries.

- `RPL_START_ROUTING_ENTRIES` determines the initial number of allocated entries, i.e. `10` now (arbitrary value)
- the number can dynamically grow up to `RPL_MAX_ROUTING_ENTRIES`
- this tackles #491