uboot-imx

MA-14712 change androidboot.storage_type to androidboot.boot_device_root

storage_type is emmc or sd, which is not friendly
for user space, for example android health HAL need access
/sys/block/mmcblkX/stat.

Replace it with mmcblkX named as boot_device_root.
(Keep storage_type for compatibility now)

Change-Id: I7486d522696e9fe3dde8bdc8834ac11d25df7d79
Signed-off-by: Jindong <jindong.yue@nxp.com>