I have not inspected the problem very thoroughly, but in short it does not work because something in the freetz-ng installation fails.
To get the freetz unpacker working without the fact_extractor you have to do:
# These variables instruct freetz to use the system versions of lzma and sfk
export UNLZMA=$(which lzma)
# http://stahlworks.com/dev/swiss-file-knife.html
export SFK=$(which sfk)
tools/unpack-kernel -u kernel.img
I have not inspected the problem very thoroughly, but in short it does not work because something in the freetz-ng installation fails. To get the freetz unpacker working without the fact_extractor you have to do: