init: Support setting statically assigned DNS server addresses
This commit introduces support for setting DNS server addresses to
lwIP's DNS subsystem. The information is found on uknetdev's extended
information fields `UK_NETDEV_IPV4_DNS0` and `UK_NETDEV_IPV4_DNS1`.
Signed-off-by: Simon Kuenzer <simon@unikraft.io> Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com> Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #42