v2017.11-libretech-cc

libretech-cc_defconfig: Add env config to be in SD

Previous libretech-cc_defconfig would not manage u-boot environment.
Enable the options to load and save the uboot.env from a FAT partition.

This works with an SD card using MSDOS partition table and a FAT
partition.

Signed-off-by: Loys Ollivier <lollivier@baylibre.com>