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/woodburn_sd_defconfig b/configs/woodburn_sd_defconfig
index 63ee104..5412500 100644
--- a/configs/woodburn_sd_defconfig
+++ b/configs/woodburn_sd_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
+CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/woodburn/imximage.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y