[coreboot] error building coreboot

[chp at posteo.de]

hi,
i am trying to build coreboot according to

https://github.com/bibanon/Coreboot-ThinkPads/wiki/T60p-Build-Coreboot#build-coreboot

make crossgcc worked fine.

after running "make" im getting the following error and the building 
process is stopped:
---------
/coreboot/util/cbfstool/cbfs_image.c:451:59: error: comparison between 
signed and unsigned integer expressions [-Werror=sign-compare]
   assert((char*)CBFS_SUBHEADER(entry) - image->buffer.data ==
                                                            ^
cc1: all warnings being treated as errors
make: *** [build/util/cbfstool/cbfs_image.o] Fehler 1
---------

any ideas what is causing this error and how to fix it?

thanks in advance

chp