arm: mvebu: turris_{omnia, mox}: Reset bootdelay env for rescue

When rescue mode was activated reset also bootdelay env variable to its
default value. This will ensure that reset button works and starts rescue
mode also in the case when user changed bootdelay env variable to -1 (which
has meaning to not start autoboot).

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