ci: Make sure that we have a proper shell prompt for LAVA.

ci: Make sure that we have a proper shell prompt for LAVA.

LAVA finds a '#' early in boot and races to emit its shell commands.
Apparently for the current boards those serial commands end up getting
buffered such that things work out, but for db410c and db820c, the buffer
is lost and LAVA gets stuck waiting for the prompt.  By setting a prompt,
we can delay our commands until we're actually supposed to emit them (and
suppress a complaint from the lava dispatcher that we're using a risky
prompt!)

Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3883>