The FF_UART is not only avilible on the large extension connector. FF_UART is also available on the three trough-hole pads close to the SD Card slot. (Limestone older than V2.0: beside the battery connector X12). The square pad is pad 1.
The voltage level of these signals is 3.3V and not compatible with the RS232 standard. You have to use RS232 level converter between the Limestone and your PC! As a workaround you could use the EvalBoard as level shifter. Connect the Limestone FF_UART pins to the EvalBoard as shown below. Remove the Colibri from the EvalBoard and power the Board. Use the lower RS232 connector of the EvalBoard.