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/omap5_uevm_defconfig b/configs/omap5_uevm_defconfig
index ec75672..4d9de61 100644
--- a/configs/omap5_uevm_defconfig
+++ b/configs/omap5_uevm_defconfig
@@ -2,6 +2,7 @@
 CONFIG_OMAP54XX=y
 CONFIG_TARGET_OMAP5_UEVM=y
 CONFIG_SPL=y
+# CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y