Convert CONFIG_SPL_ONENAND_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/igep0020_defconfig b/configs/igep0020_defconfig
index b4b6cbb..1ee8032 100644
--- a/configs/igep0020_defconfig
+++ b/configs/igep0020_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_ONENAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set