sandbox: add test of CONFIG_ENV_IMPORT_FDT

Check that a variable defined in /config/environment is found in the
run-time environment, and that clearing fdt_env_path from within that
node works.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
[trini: Conditionalize the test being linked in]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 6fd56c3..fe26ced 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -53,6 +53,13 @@
 		osd0 = "/osd";
 	};
 
+	config {
+		environment {
+			from_fdt = "yes";
+			fdt_env_path = "";
+		};
+	};
+
 	audio: audio-codec {
 		compatible = "sandbox,audio-codec";
 		#sound-dai-cells = <1>;