Convert CONFIG_CMD_ONENAND to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_ONENAND

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/smdkc100_defconfig b/configs/smdkc100_defconfig
index ef8510d..82ba51c 100644
--- a/configs/smdkc100_defconfig
+++ b/configs/smdkc100_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SYS_PROMPT="SMDKC100 # "
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
+CONFIG_CMD_ONENAND=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_FAT=y