Allow add_img_to_target_files to work on a directory

In addition to the current behavior of add_img_to_target_files working
on an existing zip file, allow passing in a directory where the target
files have already been extracted. When in this mode, it writes the
images out to that directory instead of the zip file.

This allows us to call add_img_to_target_files on the temp directory
used during the build to create the target files package, saving the
time and space of unzipping what we just zipped. This also allows us to
use the parallel soong_zip, which compresses the images much faster.

Test: aosp_marlin target_files zip is the same before/after this change
Test: marlin target_files zip is the same before/after this change
Test: bullhead target_files zip is the same before/after this change
Change-Id: I155654cdc7ad7754ba4ef76ec69c31f504a58198