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.
Merge remote-tracking branch 'origin/device_access'
Add comments explaining the use of sequence numbers to detect data loss in the example application
defconfigs: increase CONFIG_NUM_DOMAINS to match number of example cells
Resolve merge conflicts and update example application to use channels
Remove stale commented header line
Rename _puts and _putchar to use standard names, update types to match library headers
Rename cell1/cell3-specific I/O routines to avoid clashes with C library routines that may work with different devices
Merge branch 'master' of https://github.com/GaloisInc/sk-config-example into GaloisInc-master
Merge branch 'GaloisInc-master'