arm: mvebu: turris_omnia: update rescue mode boot command

Update rescue mode boot command on Turris Omnia. We are compressing the
image with lzma now.

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