The uCR core only needs to be ported to run on your board if the CPU is not yet supported. The uCR core has no device drivers, serial drivers, console interfaces or bootstrap code make code board specific. (You as the programmer put that stuff in the linker scripts.) Porting effort consists almost entirely of incorporating the thread and interrupt support for your new target.
If your CPU is supported, but uCR does not include a model file or locore.o for your board, you may supply your own. This is simpler then a complete retarget effort because all the thread handling and debugging code already exists, and you just require a linker script and locore.s source file. The section in "Link Time Context" tells much about what needs to be done in this case.