Assert the stash size when generating OTAs.

With block-based OTA v2 and v3, it requires stash space on the /cache
partition to back up blocks during an update. We need to ensure that
it doesn't exceed the partition size. Since there might be other files
on /cache as well, we use cache_size * threshold as the maximum allowed
size. The threshold defaults to 0.8, which can be overridden by command
line option '--stash_threshold'.

Change-Id: I5c2e353ea4716328d6cdc6a7f758520de8d88b8e
(cherry picked from commit 23ac4042)