ARM: tegra: make use of negative ENV_OFFSET on NVIDIA boards

Use a negative value of CONFIG_ENV_OFFSET for all NVIDIA reference boards
that store the U-Boot environment in the 2nd eMMC boot partition. This
makes U-Boot agnostic to the size of the eMMC boot partition, which can
vary depending on which eMMC device was actually stuffed into the board.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Tom Warren <twarren@nvidia.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
diff --git a/include/configs/cardhu.h b/include/configs/cardhu.h
index 6a99175..f3916de 100644
--- a/include/configs/cardhu.h
+++ b/include/configs/cardhu.h
@@ -55,7 +55,7 @@
 
 /* Environment in eMMC, at the end of 2nd "boot sector" */
 #define CONFIG_ENV_IS_IN_MMC
-#define CONFIG_ENV_OFFSET		((512 * 1024) - CONFIG_ENV_SIZE)
+#define CONFIG_ENV_OFFSET		(-CONFIG_ENV_SIZE)
 #define CONFIG_SYS_MMC_ENV_DEV		0
 #define CONFIG_SYS_MMC_ENV_PART		2