build_image.py: Make sparse squashfs images

Change-Id: Iea0399868a9406f9929d8176d61d5698a2043697
Signed-off-by: Mohamad Ayyash <mkayyash@google.com>

tools/releasetools/build_image.py

@@ -268,6 +268,7 @@ def BuildImage(in_dir, prop_dict, out_file):
   elif fs_type.startswith("squash"):
     build_command = ["mksquashfsimage.sh"]
     build_command.extend([in_dir, out_file])
+    build_command.extend(["-s"])
     build_command.extend(["-m", prop_dict["mount_point"]])
     if "selinux_fc" in prop_dict:
       build_command.extend(["-c", prop_dict["selinux_fc"]])