Free the stash after its use in BBOTA v2.

In BBOTA v2, the stash size computation is inaccurate. A stash will be
used only once, so we deduct its size from stashed_blocks right after
its use. However, the stash doesn't actually get freed until being
overwritten by another stash or destroyed at the end of an update. This
leads to OTA failures due to insufficient space on /cache. This CL
changes the BBOTA v2 script to generate an explicit free right after
its use.

Bug: 23119955
Change-Id: Icdd8352bb0ff778945937c870990a888c7a4d38b
Ticket: RENDANG-473