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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 05ee357..f6baee0 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -32,6 +32,7 @@
 CONFIG_SPL_NAND_BASE=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SYS_SPL_ARGS_ADDR=0x84000000
+CONFIG_SYS_NAND_SPL_KERNEL_OFFS=0x2a0000
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x1700
 # CONFIG_SPL_POWER is not set