split out device-specific recovery UI code into vendor directories

Take some device-specific details of the recovery UI (eg, what keys to
press to bring up the interface and perform actions, exact text of the
menu, etc.) and split them out into separate C functions.  Arrange to
take implementations of those functions from the appropriate vendor
directory at build time.  Provide a default implementation in case no
vendor-specific one is available.
6 files changed