- prework for fixing bug #45029: access IPv4 configuration of struct netif via new...

- prework for fixing bug #45029: access IPv4 configuration of struct netif via new API (netif_ip4_addr()/netif_ip4_netmask()/netif_ip4_gw()) instead of accessing the struct member directly. This way, we can change the struct member types from ip4_addr_t to ip_addr_t;
- fixed some bugs in calls to ip4_addr*() where the cast to u8_t* did not reveal the wrong address type