arm: mvebu: turris_omnia: fix rescue mode bootcmd bootargs setting

arm: mvebu: turris_omnia: fix rescue mode bootcmd bootargs setting

Rescue mode bootcmd currently only appends the "omniarescue" parameter
to the bootargs variable. We do not want the user to be able to change
rescue mode bootargs. Therefore change this so that bootcmd sets the
bootargs variable in an absolute way (adding console device information
and the omniarescue paramterer).

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