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/thuban_defconfig b/configs/thuban_defconfig
index 7995974..cf42f46 100644
--- a/configs/thuban_defconfig
+++ b/configs/thuban_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-draco"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y