Configure default DNS resolver in 6lbr example

[OlegHahm]

Contribution description

For convenience, it would be nice to have DNS resolution enabled by default in the examples. Hence, this patch configures a default DNS resolver on the 6lbr example and propagates its via RDNSS option in RAs to the nodes.

Once again the tradeoff is some increased ROM and RAM usage.

Testing procedure

1. Setup a border router (for instance on the nrf52840dongle), for instance, via BOARD=nrf52840dongle make -C examples/gnrc_border_router clean all flash ULINK=cdc-ecm.
2. Setup a second 6lowpan capable device with, for instance, gnrc_networking while enabling USEMODULE += sock_dns and USEMODULE += gnrc_ipv6_nib_dns.
3. Check if pinging a hostname works, e.g., ping riot-os.org.

Issues/PRs references

Can be tested with #21116.

[riot-ci]

Murdock results

✔️ PASSED

e668ac4 examples/gnrc_border_router: set DNS resolver

Success	Failures	Total	Runtime
19	0	19	01m:42s

Artifacts

• Documentation preview