env_mmc: allow environment to be in an eMMC partition

eMMC devices may have hardware-level partitions: 2 boot partitions,
up to 4 general partitions, plus the user area. This change introduces
optional config variable CONFIG_SYS_MMC_ENV_PART to indicate which
partition the environment should be stored in: 0=user, 1=boot0, 2=boot1,
4..7=general0..3. This allows the environment to be kept out of the user
area, which simplifies the management of OS-/user-level (MBR/GPT)
partitions within the user area.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>