tegra: put eMMC environment into the boot sectors

When I set up Tegra's config files to put the environment into eMMC, I
assumed that CONFIG_ENV_OFFSET was a linearized address relative to the
start of the eMMC device, and spanning HW partitions boot0, boot1,
general* and the user area in order. However, it turns out that the
offset is actually relative to the beginning of the user area. Hence,
the environment block ended up in a different location to expected and
documented.

Set CONFIG_SYS_MMC_ENV_PART=2 (boot1) to solve this, and adjust
CONFIG_ENV_OFFSET to be relative to the start of boot1, not the entire
eMMC.

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