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/configs/mx6ul_14x14_evk_defconfig b/configs/mx6ul_14x14_evk_defconfig
index afebc60..ac1c26d 100644
--- a/configs/mx6ul_14x14_evk_defconfig
+++ b/configs/mx6ul_14x14_evk_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
+CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y