ENGR00320125 iMX6: Change to use dynamical mmcroot

The kernel changes to fix the mmcblk index with USDHC controllers
as below:
mmcblk0 ---> USDHC1
mmcblk1 ---> USDHC2
mmcblk2 ---> USDHC3
mmcblk3 ---> USDHC4

So in u-boot, the "mmcroot" must be updated together.

When booting from SD/MMC device, change the "mmcroot" to
dynamically set according to the boot USDHC controller.
It is the same mechanism as "mmcdev" used for kernel image loading.
Therefore, the uboot, kernel image, dtb and rootfs are required
in same SD/MMC card.
To disable the mmc dynamical detection, set the "mmcautodetect" to "no",
then "mmcroot" and "mmcdev" will not be overwritten.

When booting from other devices which needs to load kernel, dtb and
rootfs from SD/MMC card, their "mmcdev" reset vaule is
CONFIG_SYS_MMC_ENV_DEV and "mmcroot" reset value is CONFIG_MMCROOT.

Signed-off-by: Ye.Li <B37916@freescale.com>

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