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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index ce79df4..6fd6fb1 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -1,13 +1,14 @@
 CONFIG_ARM=y
 CONFIG_OMAP34XX=y
+# CONFIG_SPL_GPIO_SUPPORT is not set
+# CONFIG_SPL_EXT_SUPPORT is not set
 CONFIG_TARGET_AM3517_EVM=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_BOOTDELAY=10
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
-# CONFIG_SPL_EXT_SUPPORT is not set
-# CONFIG_SPL_GPIO_SUPPORT is not set
+CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="AM3517_EVM # "
 CONFIG_CMD_BOOTZ=y