View source for RaspberryPi: TNC Packet Radio

Beyond what others have mentioned like the TNC-Pi (nice kit), using a $7 USB soundcard and Direwolf will provide superior decodes.  As for OS images to run, that are packet centric, there are several available:

* Bernard's new Jessie based image for TNC-Pi - http://f6bvp.org/AX25_BBS_Node_RaspBerry_Pi_install.html
* Mitch's Wheezy based image with setup GUI which supports TNC-X and Direwolf - https://sourceforge.net/projects/haldigital/
* I've been working on my own documentation for such a thing (v2 on Jessie is close but incomplete, v1 for Wheezy is done) for Direwolf and TNC-Pi - http://www.trinityos.com/HAM/CentosDigitalModes/RPi/
* Brett's TNC-Pi centric image - https://sites.google.com/a/thinkonit.com/ax25-fpac/