releasetools: Allow logging the diff for incrementals.

Add an option "--log_diff <filename>" to ota_from_target_files.py
script. When enabled, it logs the differences between the source
and target builds into <filename> when generating incremental OTAs.

Also move target_files_diff.py into releasetools/ so that it can be
packed into otatools.zip.

Bug: 25372309
Change-Id: Ifd4ed0f2f12ef040ee377621ec8c35a873cec34f