Convert CONFIG_SPL_NAND_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/duovero_defconfig b/configs/duovero_defconfig
index 96218ff..64f9cbc 100644
--- a/configs/duovero_defconfig
+++ b/configs/duovero_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_OMAP44XX=y
 # CONFIG_SPL_I2C_SUPPORT is not set
+# CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_TARGET_DUOVERO=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y