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/omap3_beagle_defconfig b/configs/omap3_beagle_defconfig
index 176ce96..ece9638 100644
--- a/configs/omap3_beagle_defconfig
+++ b/configs/omap3_beagle_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
+CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set