This contains functionality for talking to UART/Serial devices on different platforms. The official client will build either uart_posix.c and uart_win32.c. Build targets for these files are contained in client/Makefile.
If you want to implement support for other platforms, you need to implement the methods provided in uart.h.
Each driver is called with a string, typically containing a path or other reference to a serial port on the host. The methods outlined in uart.h need to be implemented.
The hardware uses common/usb_cdc.c to implement a USB CDC endpoint exposed by the Atmel MCU.