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

Quick walkthrough of the steps needed for building:

1) mkdir -pv build; cd build - this will enter a build directory, as an in-dir build is not supported
2) cmake -DCMAKE_TOOLCHAIN_FILE=../toolchains/arm-none-eabi.cmake ..
3) make
4) Kernel is in kernel/kernel