Convert CONFIG_SYS_NAND_SPL_KERNEL_OFFS to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_NAND_SPL_KERNEL_OFFS

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/igep00x0_defconfig b/configs/igep00x0_defconfig
index 96f2f9d..3b65a22 100644
--- a/configs/igep00x0_defconfig
+++ b/configs/igep00x0_defconfig
@@ -42,6 +42,7 @@
 CONFIG_SPL_ONENAND_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SYS_SPL_ARGS_ADDR=0x84000000
+CONFIG_SYS_NAND_SPL_KERNEL_OFFS=0x280000
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x1700
 CONFIG_SYS_MAXARGS=64