configs: e850-96: Enable U-Boot environment on eMMC

configs: e850-96: Enable U-Boot environment on eMMC

Store U-Boot environment in BOOT2 HW area of eMMC (/dev/mmcblk0boot1),
as it's currently unused. BOOT1 area will be probably used for storing
low-level bootloaders further, so let's not touch it.

Both primary and redundant environments work fine:

    => env default -f -a
    ## Resetting to default environment
    => env save
    Saving Environment to MMC... Writing to redundant MMC(0)... OK

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>