minor recovery changes

- add the --just_exit option to make recovery exit normally without doing anything
- make it possible to build updater extensions in C++
- add the clear_display command so that the updater binary can request
  recovery switch to the NONE background UI

These are all used to support the notion of using OTA as a factory
reflash mechanism.

Change-Id: Ib00d1cbf540feff38f52a61a2cf198915b48488c