Convert CONFIG_SPL_MTD_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/am335x_igep0033_defconfig b/configs/am335x_igep0033_defconfig
index 296e8a4..2b90a00 100644
--- a/configs/am335x_igep0033_defconfig
+++ b/configs/am335x_igep0033_defconfig
@@ -9,9 +9,10 @@
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
+CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
-CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set