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.
I had thought the build system (e.g. the cells' Makefile) knew to include libs/libskc on the include path, and so this symlink wouldn't be necessary.