fix fstab generation

Change-Id: I223a07fd0ce3c0d48b7f18f5c42ae204affe087a
diff --git a/roots.c b/roots.c
index 39179e9..19855b2 100644
--- a/roots.c
+++ b/roots.c
@@ -302,12 +302,13 @@
 get_root_partition_device(const char *root_path, char *device)
 {
     const RootInfo *info = get_root_info_for_path(root_path);
-    if (info == NULL || info->device != g_default_device ||
-            info->partition_name == NULL)
+    if (info == NULL)
     {
         return NULL;
     }
-    return get_partition_device(info->partition_name, device);
+    if (info->device == g_default_device)
+        return get_partition_device(info->partition_name, device);
+    return info->device;
 }
 
 #ifndef BOARD_HAS_NO_MISC_PARTITION