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>