AVM urlader environment stored in the device tree and exposed via
/proc/sys/urlader/environment.

Required properties:
 - compatible            : "avm,dt-urlader-env"
 - avm,urlader-env-names : Whitelist of environment variables to expose
                           if they are stored in the device tree under
                           /chosen/${name}

Example:
	dt-urlader-env {
		compatible = "avm,dt-urlader-env";

		avm,urlader-env-names = "HWRevision", "HWSubRevision", "annex",
					"bootloaderVersion", "maca", "macb",
					"macdsl", "gpon_serial";
	};