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/am335x_guardian_defconfig b/configs/am335x_guardian_defconfig
index 658155f..fe8ddf6 100644
--- a/configs/am335x_guardian_defconfig
+++ b/configs/am335x_guardian_defconfig
@@ -64,6 +64,7 @@
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_SPL_ENV_IS_NOWHERE=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_BOOTP_SEND_HOSTNAME=y