arm: mvebu: turris_omnia: Add CONFIG_BOARD_SIZE_LIMIT

Maximal size of u-boot kwb image binary is $CONFIG_ENV_OFFSET which is
0xF0000 = 983040 bytes. So add missing CONFIG_BOARD_SIZE_LIMIT definition
to ensure that u-boot binary does not overflow to the u-boot env storage.

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