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/igep0030_nand_defconfig b/configs/igep0030_nand_defconfig
index c8f875e..58a03b9 100644
--- a/configs/igep0030_nand_defconfig
+++ b/configs/igep0030_nand_defconfig
@@ -1,10 +1,11 @@
 CONFIG_ARM=y
 CONFIG_OMAP34XX=y
+# CONFIG_SPL_EXT_SUPPORT is not set
 CONFIG_TARGET_OMAP3_IGEP00X0=y
 CONFIG_SYS_EXTRA_OPTIONS="MACH_TYPE=MACH_TYPE_IGEP0030,BOOT_NAND"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
-# CONFIG_SPL_EXT_SUPPORT is not set
+CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set