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.
Note that as of 6e4a48195fc2a66b229ec54672a6dbd84df78d2d the cells no longer use symlinks.