ci: Make a simple little bare-metal fastboot mode for db410c.

ci: Make a simple little bare-metal fastboot mode for db410c.

This supports powering up the device (using an external tool you
provide based on your particular lab), talking over serial to wait for
the fastboot prompt, and then booting a fastboot image on a target
device.

I was previously relying on LAVA for this, but that ran afoul of
corporate policies related to the AGPL.  However, LAVA wasn't doing
too much for us, given that gitlab already has a job scheduler and
tagging and runners.  We were spending a lot of engineering on making
the two systems match up, when we can just have gitlab do it directly.

Lightly-reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4076>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4076>