The autopilot includes:
- Linux 3.8 kernel compiled with the PREEMPT option (best results we measured during Beaglepilot)
- Debian Wheezy file system
- ROS Hydromedusa
- mavros ROS package
- APM running natively in Linux (and linked with ROS through mavros)
- preconfigured daemons for launching everything automatically, certain WiFi dongles support (modules for other could be compiled though)
There’s an active discussion about the autopilot here.