releasetools: add compatibility for full ota functions with incrementals

Some device-specific releasetool functions may expect that input_zip
and input_version are set. For incremental OTAs, target_zip and
target_version are set instead.

Set input_zip=target_zip and input_version=target_version to add
compatibility with these functions.

Change-Id: I6a04f67440618d3652396656cc1fe223d4a6b195