[TASK] Integrate boardfarm in gitlab-ci

[arnout]

The script tests/run_bf.sh runs boardfarm. This should also be done from CI.

• Create a Dockerfile with boardfarm
• Build this in CI
• Run boardfarm in CI
• Run boardfarm on prpWrt device (Port test_initial_ap_config to boardfarm for prplWRT device #1072) in CI

For the latter, the docker in CI has to be able to bridge to the external network. It needs to be investigated how to do that with a gitlab-runner.