A simple routing table implementation for addition, deletion and lookup of IPv6 routes.ﾂ

APIs are:
1) s8_t ip6_add_route_entry(struct ip6_prefix *ip6_prefix,
ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ struct netif *netif,
ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ip6_addr_t *gateway,
ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ ﾂ s8_t *index);

2) err_t ip6_remove_route_entry(struct ip6_prefix *ip6_prefix);

3) s8_t ip6_find_route_entry(ip6_addr_t *ip6_dest_addr);

4) struct netif *ip6_static_route(ip6_addr_t *src, ip6_addr_t *dest);

5) ip6_addr_t *ip6_get_gateway(struct netif *netif, ip6_addr_t *dest);

6) struct ip6_route_entry *ip6_get_route_table(void);

For route lookup from the table, TheﾂLWIP_HOOK_IP6_ROUTE hook in ip6_route(..) of ip6.c
could be assigned to the ip6_static_route() API of this implementation to return the 
appropriate netif.

-- The application can add routes using the API ip6_add_route_entry(..).ﾂ
ﾂ ﾂThis API adds the ip6 prefix route into the static route table while
ﾂ ﾂkeeping all entries sorted in decreasing order of prefix length.
ﾂ ﾂSubsequently, a linear search down the list can be performed to retrieve a
ﾂ ﾂmatching route entry for a Longest Prefix Match.
ﾂ ﾂThe prefix length is expected to be at an 8-bit boundary. While this isﾂ
ﾂ ﾂa limitation, it would serve most practical purposes.

-- The application can remove routes using the API ip6_remove_route_entry(..).

-- The application can find a route entry for a specific address using theﾂ
ﾂ ﾂip6_find_route_entry() function which returns the index of the found entry.ﾂ
ﾂ ﾂThis is used internally by the route lookup function ip6_static_route() API.

-- To fetch the gateway IPv6 address for a specific destination IPv6ﾂ
ﾂ ﾂaddress and target netif, the application can call ip6_get_gateway(..).
   This API could be assigned to the LWIP_HOOK_ND6_GET_GW() if a gateway has
   been added as part of the ip6_add_route_entry().

-- To fetch a pointer to the head of the table, the application can callﾂ
ﾂ ﾂip6_get_route_table().