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/igep0020_defconfig b/configs/igep0020_defconfig
index 8ca5a84..2375f86 100644
--- a/configs/igep0020_defconfig
+++ b/configs/igep0020_defconfig
@@ -20,6 +20,7 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_NAND=y
+CONFIG_CMD_ONENAND=y
 CONFIG_CMD_SPI=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_GPIO=y