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>
diff --git a/configs/mx51evk_defconfig b/configs/mx51evk_defconfig
index 1505459..a2af8ae 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -3,7 +3,7 @@
 CONFIG_SYS_TEXT_BASE=0x97800000
 CONFIG_TARGET_MX51EVK=y
 CONFIG_ENV_SIZE=0x2000
-CONFIG_ENV_OFFSET=0x60000
+CONFIG_ENV_OFFSET=0xC0000
 CONFIG_NR_DRAM_BANKS=1
 # CONFIG_CMD_BMODE is not set
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx51evk/imximage.cfg"