releasetools: Support packaging for system_root_image.

For system images that contain the root directory, we need to find the
root directory at ROOT/ instead of BOOT/RAMDISK/.

Change-Id: Ica345c8b1b03475f8ac6c44fd576045fcf17c882
diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index 69987ab..03b7e05 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -617,8 +617,8 @@
     symlinks = CopyPartitionFiles(system_items, input_zip, output_zip)
     script.MakeSymlinks(symlinks)
 
-  boot_img = common.GetBootableImage("boot.img", "boot.img",
-                                     OPTIONS.input_tmp, "BOOT")
+  boot_img = common.GetBootableImage(
+      "boot.img", "boot.img", OPTIONS.input_tmp, "BOOT")
 
   if not block_based:
     def output_sink(fn, data):