Convert CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/colibri-imx8x_defconfig b/configs/colibri-imx8x_defconfig
index ca966a9..d9c1e4d 100644
--- a/configs/colibri-imx8x_defconfig
+++ b/configs/colibri-imx8x_defconfig
@@ -33,6 +33,7 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_MMC_ENV_PART=1
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_IP_DEFRAG=y
 CONFIG_TFTP_BLOCKSIZE=4096