lava: Work around LAVA 2021.05 regression with kernel_start_message

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

Apparently `kernel_start_message` is not working correctly in the
`context` section, so move it to the relevant `boot` sections.

It may also be related that the correct spelling is
`kernel-start-message` so I am not sure about anything anymore.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>