mmc: Update environment variable with active partition

This patch allows uboot scripts make choices about where to boot from based
on the active mmc boot partition. This allows having two copies of kernel,
filesystems etc, and choosing which to boot from based off of the active
bootloader partition.

Signed-off-by: Reuben Dowle <reuben.dowle@4rf.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>