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/mx6sxsabresd_spl_defconfig b/configs/mx6sxsabresd_spl_defconfig
index 9c03546..7b408e2 100644
--- a/configs/mx6sxsabresd_spl_defconfig
+++ b/configs/mx6sxsabresd_spl_defconfig
@@ -40,6 +40,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_DM_GPIO=y
 CONFIG_DM_I2C=y
 CONFIG_DM_MMC=y