arm: mvebu: turris_omnia: support invoking rescue boot from console

Make it possible to invoke rescue boot from U-Boot console, without
having to press the factory reset button. This is needed when accessing
the device remotely, for example.

Achieve this by putting rescue command into `bootcmd_rescue` default
environment variable and setting some distroboot environment variables
to their default values when the factory button is pressed.

Rescue boot from console can be invoked by running
  run bootcmd_rescue

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>