ota_from_target_files: allow cow version override

With our version compatibility check, overriding our cow version no
longer works. Updating this check to only apply if a command line
vabc_cow_version is not specified.

NOTE: the ota may not successfully apply, depending on device if we
override the cow version

Test: th
Change-Id: Ibbed6cf94cc2e91597d0c249dc8ade314b8341a2