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/imx8qm_mek_defconfig b/configs/imx8qm_mek_defconfig
index 1aca217..01ba6cf 100644
--- a/configs/imx8qm_mek_defconfig
+++ b/configs/imx8qm_mek_defconfig
@@ -50,6 +50,7 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_MMC_ENV_DEV=1
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_SPL_DM=y
 CONFIG_SPL_CLK=y
 CONFIG_CLK_IMX8=y