mx51evk: Fix U-Boot corruption after saving the environment

U-Boot binary has grown in such a way that it goes beyond the reserved
area for the environment variables.

Running "saveenv" followed by a "reset" causes U-Boot to hang because
of this overlap.

Fix this problem by increasing the CONFIG_ENV_OFFSET size.

Also, in order to prevent this same problem to happen in the future,
use CONFIG_BOARD_SIZE_LIMIT, which will detect the overlap in build-time.

CONFIG_BOARD_SIZE_LIMIT does not accept math expressions, so declare
CONFIG_ENV_OFFSET with its direct value instead.

Signed-off-by: Fabio Estevam <festevam@gmail.com>