Convert CONFIG_SPL_MMC_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/cm_fx6.h b/include/configs/cm_fx6.h
index c839b03..6a0c2af 100644
--- a/include/configs/cm_fx6.h
+++ b/include/configs/cm_fx6.h
@@ -247,9 +247,8 @@
 
 /* SPL */
 #include "imx6_spl.h"
-#define CONFIG_SPL_MMC_SUPPORT
+#undef CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR
 #define CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR	0x80 /* offset 64 kb */
-#define CONFIG_SYS_MONITOR_LEN	(CONFIG_SYS_U_BOOT_MAX_SIZE_SECTORS / 2 * 1024)
 #define CONFIG_SPL_SPI_SUPPORT
 #define CONFIG_SPL_SPI_FLASH_SUPPORT
 #define CONFIG_SYS_SPI_U_BOOT_OFFS	(64 * 1024)