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/brxre1_defconfig b/configs/brxre1_defconfig
index 4547605..fe1fbb6 100644
--- a/configs/brxre1_defconfig
+++ b/configs/brxre1_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1"
 CONFIG_BOOTDELAY=-2
 CONFIG_VERSION_VARIABLE=y