ci: Switch testing on db410c over to LAVA.

ci: Switch testing on db410c over to LAVA.

This should get us better stability of the db410c boards by having a
smaller per-board software stack, with no disks involved (just initramfs).
Additionally, the new cluster is 7 (soon 8) db410cs, while currently the
docker cluster only has 1/4 of its db410cs still running.

Unfortunately, we have to prepare the fastboot boot image during the ARM
drivers build stage, because LAVA relies on publicly available URLs for
the images to load into the bootloaders of the boards, and the only thing
we have for that is gitlab's artifacts.

Note that this testing relies on the boards being freshly flashed with the
linaro v136 firmware to pick up the initramfs size fixes and to stop the
boot at fastboot.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3661>