Adds --output-super-empty flag to enable building super_empty.img.

Any mixed build that uses dynamic partitions will require a
super_empty.img image. This image must be created from the merged
misc_info.txt file, so adding this functionality here simplifies
the creation of this image for users (versus having to call
build_super_image.py manually after calling merge_target_files.py).

Bug: 129976345
Test: Ran merge_target_files.py on a dynamic partition enabled build
using the new --output-super-empty flag.

Change-Id: I73901f363d73c9fae1af1579faa2a908369dbbec