Adds --output-img flag to enable building the IMG package.

This simplifies the use case for mixed build users. Instead of having to
remember to call img_from_target_files.py after this script, they can
use this flag to automatically create the IMG package.

Also includes an update to super_empty.img logic. The super_empty.img is
now always created for dynamic-partition builds. The flag now only
controls copying the super_empty.img to a user-provided location.

Bug: 129976345
Test: Ran merge_target_files.py using --output-img and
--output-super-empty and inspected the resulting img zip and
super_empty.img.

Change-Id: Ida602942bb7a6b4b94f4e225640af9104fc9360c