tegra: whistler: store environment in eMMC

tegra: whistler: store environment in eMMC

Store the environment in eMMC, at the end of the second boot sector.
This should not conflict with any other eMMC usage: U-Boot is stored
well below this location, and the kernel only uses the general area
of the eMMC once booted, not the boot sectors.

Note: This assumes the user plugged the standard 8MB MoviNAND card into
J29/HSMMC/POP. If they didn't, the boot sector layout may be different.
However, use of that particular card is standard practice as far as I
know.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>