Update sk-config example applications to show serial-to-serial communication

This change adds a third cell and refactors the system so that user
input originating from UART2 on the Sabre Lite is read by cell 1,
processed (and potentially modified) by cell 2, and emitted by cell
3 on UART1.
Yeah, I couldn't tell what I needed to do here. I added the symlinks because it looked like the others were under revision control, but I definitely had the feeling that it might not be the right thing to do. What should I do about this?