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_evm_defconfig b/configs/am335x_evm_defconfig
index f88c4a0..1d92bb8 100644
--- a/configs/am335x_evm_defconfig
+++ b/configs/am335x_evm_defconfig
@@ -8,6 +8,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
+CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set