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/devkit8000_defconfig b/configs/devkit8000_defconfig
index dc07c14..f8a9853 100644
--- a/configs/devkit8000_defconfig
+++ b/configs/devkit8000_defconfig
@@ -23,6 +23,7 @@
 CONFIG_SPL_NAND_BASE=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SYS_SPL_ARGS_ADDR=0x80000100
+CONFIG_SYS_NAND_SPL_KERNEL_OFFS=0x280000
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x500
 CONFIG_SYS_MAXARGS=64