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/am43xx_evm_defconfig b/configs/am43xx_evm_defconfig
index ebfb018..0825ea1 100644
--- a/configs/am43xx_evm_defconfig
+++ b/configs/am43xx_evm_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SPL_NET=y
 CONFIG_SPL_NET_VCI_STRING="AM43xx U-Boot SPL"
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SYS_NAND_SPL_KERNEL_OFFS=0x300000
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x1700
 CONFIG_SPL_USB_HOST=y