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.
Sorry, I think this confusion was introduced by myself. At the time this was originally implemented, the build system staged all libraries required by any app in the same place and expected them to namespace themselves. As a result, stddef.h from Musl C and stddef.h from libskc collided. If I remember correctly, this symlink was an attempt to work around this. Ultimately this (single, unmanaged library header staging area) is an issue with the general seL4 build system that should be resolved there, but we may need to keep doing things like this to work around it for the immediate future.