ArduPilot now uses the AP_Bootloader in the main ArduPilot git repository.
For more information see this file:
https://github.com/ArduPilot/ardupilot/blob/master/Tools/AP_Bootloader/README.md
The board_types.txt files is now kept here:
https://github.com/ArduPilot/ardupilot/blob/master/Tools/AP_Bootloader/board_types.txt