iMX6: Change mmcroot to use fixed mmc block index for USDHC

Kernel changed to use fixed mmcblk index for USDHC, which is
mmcblk0 --> USDHC1
mmcblk1 --> USDHC2
mmcblk2 --> USDHC3
mmcblk3 --> USDHC4
Thus, change the default mmcroot in u-boot to fit the changes in kernel.

Signed-off-by: Ye.Li <Ye.Li@freescale.com>
wandboard: Changed mmcroot variable so kernel 3.10.31 boots

Signed-off-by: Alfonso Tamés <alfonso@tames.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>