Convert CONFIG_CMD_SPL_NAND_OFS to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SPL_NAND_OFS

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 2375f86..576674c 100644
--- a/configs/igep0020_defconfig
+++ b/configs/igep0020_defconfig
@@ -16,6 +16,7 @@
 CONFIG_SPL_ONENAND_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
 # CONFIG_CMD_IMLS is not set
+CONFIG_CMD_SPL=y
 CONFIG_CMD_ASKENV=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y