part: check CONFIG_IS_ENABLED(ENV_SUPPORT)

In SPL environment variables may not be enabled.

Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/disk/part.c b/disk/part.c
index 186ee96..eec02f5 100644
--- a/disk/part.c
+++ b/disk/part.c
@@ -508,9 +508,11 @@
 #endif
 
 	/* If no dev_part_str, use bootdevice environment variable */
-	if (!dev_part_str || !strlen(dev_part_str) ||
-	    !strcmp(dev_part_str, "-"))
-		dev_part_str = env_get("bootdevice");
+	if (CONFIG_IS_ENABLED(ENV_SUPPORT)) {
+		if (!dev_part_str || !strlen(dev_part_str) ||
+		    !strcmp(dev_part_str, "-"))
+			dev_part_str = env_get("bootdevice");
+	}
 
 	/* If still no dev_part_str, it's an error */
 	if (!dev_part_str) {