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/rpi_0_w_defconfig b/configs/rpi_0_w_defconfig
index 3570dc9..fcc2ae6 100644
--- a/configs/rpi_0_w_defconfig
+++ b/configs/rpi_0_w_defconfig
@@ -15,6 +15,7 @@
 CONFIG_OF_EMBED=y
 CONFIG_ENV_FAT_INTERFACE="mmc"
 CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_DM_KEYBOARD=y
 CONFIG_DM_MMC=y
 CONFIG_MMC_SDHCI=y