arm: mvebu: turris_{omnia, mox}: ensure running bootcmd_rescue always works

One of the points of putting the rescue boot command into default
environment is that user can invoke it without physical access to the
board (without having to press the factory reset button), by running
  run bootcmd_rescue
in U-Boot's console.

Therefore we have to ensure that bootcmd_rescue is always set to default
value, regardless of whether the factory reset button was pressed.
Otherwise the variable will be empty for example after upgrade from
previous U-Boot.

Fixes: ec3784d6 ("arm: mvebu: turris_mox: add support for board rescue mode")
Fixes: 176c3e77 ("arm: mvebu: turris_omnia: support invoking rescue boot from console")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>