releasetools: Support generating verify packages.

We can generate a special OTA package that verifies all the partitions
(boot, recovery, system, vendor and etc) on a device. It also calls
device-specific script to verify bootloader and radio images. This
ensures a flashed device contains all the desired images faithfully.

Usage:
ota_from_target_files.py --gen_verify target_files.zip output.zip

Bug: 24679956
Change-Id: Ib3091d98c4b17a6fad305b3edf16d09efbda5c38