releasetools: Always create IMAGES/ directory.

AddImagesToTargetFiles() takes either a zip file, or a zip root as
input. We used to create IMAGES/ directory only when working with zip
root input. Commit 262bf3f0 has changed
to also stage boot / recovery images there when working with a zip file.
This CL makes sure the directory is always available under both modes.

Bug: 63456822
Test: zip -d target_files.zip IMAGES/\*;
      add_img_to_target_files.py target_files.zip
Test: sign_target_files_apks.py target_files.zip signed-target_files.zip
Change-Id: Iea91d0403cdec1b16bb93bb71d3ed06856b8f7c3