ci/bare-metal: Use python for handling fastboot booting and parsing

ci/bare-metal: Use python for handling fastboot booting and parsing

Modeling after what I did for cros_servo_run.py, this gives us easy
support for restarting the test run a530 when we detect a spontaneous
reboot.  I had to touch up serial_buffer.py to handle buffering in from a
file instead of a serial device, to support the upcoming etnaviv CI
(tested by running it against a serial log from db410c and seeing it step
to calling "fastboot")

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6529>