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/am335x_evm_usbspl_defconfig b/configs/am335x_evm_usbspl_defconfig
index 5678e6a..e029717 100644
--- a/configs/am335x_evm_usbspl_defconfig
+++ b/configs/am335x_evm_usbspl_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_MUSB_NEW_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set