Adds --output-ota flag to enable building the OTA package.

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

Bug: 129976345
Test: Ran merge_target_files.py using --output-ota and inspected the
resulting zip.

Change-Id: Icc95943c24b8f83b3221e845a7d69a34c1edb4fc