Fix brokeness on Minnowboards after upgrade to LAVA 2021.05

With the update to LAVA 2021.05 jobs on the Minnowboard failed:

Setting prompt string to ['Booting in blind mode', 'error: missing (.*) symbol.']
...
Apertis v2020 apertis ttyS0
auto-login-action timed out after 853 seconds

This MR applies some fixes suggested by @sjoerd.