releasetools: Always write the last block if it's padded.

In BBOTAs if the last block of a DataImage is padded, we should always
write the whole block even for incremental OTAs. Because otherwise the
last block may be skipped if unchanged, but would fail the post-install
verification if it has non-zero contents in the padding bytes.

Bug: 23828506
Change-Id: I4b0af7344d18261258cd48d18c029c089d6ff365