imx: mx6slevk: Change default mmcdev to USDHC2 device

Since USDHC1 and USDHC3 added, the dev index for USDHC2 changed to
1. So modify the default mmcdev in environment variables to dev 1.
Signed-off-by: NYe.Li <B37916@freescale.com>

include/configs/mx6slevk.h

@@ -87,7 +87,7 @@
 	"fdt_addr=0x88000000\0" \
 	"boot_fdt=try\0" \
 	"ip_dyn=yes\0" \
-	"mmcdev=0\0" \
+	"mmcdev=1\0" \
 	"mmcpart=1\0" \
 	"mmcroot=/dev/mmcblk0p2 rootwait rw\0" \
 	"mmcargs=setenv bootargs console=${console},${baudrate} " \