Migrate CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG to Kconfig

Convert CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG to Kconfig

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Reviewed-by: Petr Vorel <petr.vorel at gmail.com>
diff --git a/configs/am335x_evm_usbspl_defconfig b/configs/am335x_evm_usbspl_defconfig
index 51415c0..25d01bb 100644
--- a/configs/am335x_evm_usbspl_defconfig
+++ b/configs/am335x_evm_usbspl_defconfig
@@ -28,6 +28,7 @@
 CONFIG_CMD_MTDPARTS=y
 CONFIG_MTDIDS_DEFAULT="nand0=nand.0"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=nand.0:128k(NAND.SPL),128k(NAND.SPL.backup1),128k(NAND.SPL.backup2),128k(NAND.SPL.backup3),256k(NAND.u-boot-spl-os),1m(NAND.u-boot),128k(NAND.u-boot-env),128k(NAND.u-boot-env.backup1),8m(NAND.kernel),-(NAND.file-system)"
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_NETCONSOLE=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_DFU_MMC=y