Summary:
As title, add capability in flashy to support GT.
TBD, once we are in DVT, we need to convert this
to flash.FlashCpVboot.

Test Plan: Build ephemeral fbpkg and test out oobgrader. Ensure it upgrades flash1.

Reviewed By: kawmarco

fbshipit-source-id: ba1c4404eb5b34cdfba3b5647ef1ec4d104e6380