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/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 74936d7..884cc8c 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -45,6 +45,7 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=spi0.0:768k(uboot),256k(uboot-environment),-(reserved)"
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_DWC_AHSATA=y
 CONFIG_DM_KEYBOARD=y
 CONFIG_DM_MMC=y