lava: Don't redeploy the overlay on reboots for aum tests

Deploy the overlay only on the first boot, rather than re-deploying it
every time we reboot.

We deploy it under `/var` so it is preserved over reboots: re-deploying
only makes the tests more time consuming.

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