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/sniper_defconfig b/configs/sniper_defconfig
index bb3d83f..7c5be7e 100644
--- a/configs/sniper_defconfig
+++ b/configs/sniper_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_SNIPER=y
 CONFIG_SPL=y
 # CONFIG_SPL_EXT_SUPPORT is not set
+# CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="sniper # "
 CONFIG_CMD_BOOTZ=y