imx: Fix regression with CONFIG_DM_MMC=y

When CONFIG_DM_MMC=y, CONFIG_BLK should be selected, otherwise the
SD/eMMC card cannot be used.

Also, select CONFIG_DM_USB=y when CONFIG_USB=y to avoid build failure.

Tested on mx6slevk, mx7dsabresd and mx6ullevk.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Adam Ford <aford173@gmail.com>
Tested-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Tested-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
diff --git a/include/configs/opos6uldev.h b/include/configs/opos6uldev.h
index d018984..04fc602 100644
--- a/include/configs/opos6uldev.h
+++ b/include/configs/opos6uldev.h
@@ -17,6 +17,7 @@
 #ifdef CONFIG_SPL_BUILD
 #undef CONFIG_DM_GPIO
 #undef CONFIG_DM_MMC
+#undef CONFIG_BLK
 
 #define CONFIG_MXC_UART_BASE		UART1_BASE
 #endif