to build mtgos you need the cross compilers, whose makefile is included in cross/

Quick walkthrough of the steps needed for building:

1) ./config.py - this will ask you a couple questions needed for building and create a appropriate config.cmake and config.h
2) mkdir -pv build; cd build - this will enter a build directory, as an in-dir build is not supported
3) cmake -DCMAKE_TOOLCHAIN_FILE=../toolchains/<toolchain name here>.cmake ..
4) make
5) Kernel is in kernel/kernel