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

tools/releasetools/img_from_target_files.py

@@ -101,6 +101,7 @@ def main(argv):
           "boot.img", "boot.img", OPTIONS.input_tmp, "BOOT")
       if boot_image:
         boot_image.AddToZip(output_zip)
+
       recovery_image = common.GetBootableImage(
           "recovery.img", "recovery.img", OPTIONS.input_tmp, "RECOVERY")
       if recovery_image: