uboot-imx

openrd: fail build if U-Boot would overlap with environment in flash

Set CONFIG_BOARD_SIZE_LIMIT so we'll notice at build time if U-Boot
has grown so large that it would overlap with the environment area in
flash, rather than bricking the device at run-time on first saveenv.

Signed-off-by: Sascha Silbe <t-uboot@infra-silbe.de>