Enable the DNS resolution in examples by default

[OlegHahm]

Contribution description

The example applications gnrc_networking and paho-mqtt are quite useful for testing and/or demonstrating RIOT's networking capabilities. For convenience the use of hostnames instead of IP addresses is preferable. So far, DNS resolution has been disabled by default. IMO it would be much nicer - particularly for newbies, if DNS resolution would work out of the box.

The downside: approx. 3k more ROM and about 160 Bytes more RAM usage (on ARM-Cortex). So, I expect this PR to fail on some constrained platforms. However, I think for an example application this is still bearable.

Testing procedure

Test with native

1. Setup an upstream device for the tap bridge (for instance via dist/tools/tapsetup/tapsetup -c 1 -u eth0).
2. Build and run paho-mqtt example via make -C example/paho-mqtt clean all term.
3. Check if you can connect to the Mosquitto test broker via con test.mosquitto.org

Test on 6lowpan hardware

1. Setup a border router with DNS resolver configured.
2. Build and run paho-mqtt example for a board, for instance via BOARD=nrf52840dk make -C example/paho-mqtt clean all flash term.
3. Check if you can connect to the Mosquitto test broker via con test.mosquitto.org

Issues/PRs references

Another PR which configures a DNS resolver on the 6lbr example by default is about to come.

[riot-ci]

Murdock results

✔️ PASSED

9f15342 paho-mqtt: enable DNS client

Success	Failures	Total	Runtime
58	0	58	01m:46s

Artifacts

• Documentation preview