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/am335x_igep003x_defconfig b/configs/am335x_igep003x_defconfig
index f640cc4..78469d7 100644
--- a/configs/am335x_igep003x_defconfig
+++ b/configs/am335x_igep003x_defconfig
@@ -40,6 +40,7 @@
 CONFIG_SPL_UBI_LOAD_KERNEL_ID=3
 CONFIG_SPL_UBI_LOAD_ARGS_ID=4
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SYS_NAND_SPL_KERNEL_OFFS=0x280000
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x1700
 CONFIG_SPL_POWER=y