Generate partial ota update package if the board defines it

Many partners have asked for platform support of system-only update.
So we config cuttlefish as an example to support the partial ota
updates. Also make such package available on the build server.
This allows continuous test to ensure the e2e update flow is working.

Bug: 170921953
Test: generate & apply a partial update, check output in presubmit
Change-Id: I79d0abeb1b2be18e6ff88f0455b6de6540a37794