You're free to read the code and compile it yourself, I don't understand this question.
That ship sailed a long time ago.
Look, being able to look at the source of truth is the best you can hope for. Moreover, the project has been around for some time. If that's not enough, then fair, at the end of the day the choice is yours.
For more detail, I point you toward the fine documentation
* git clone https://github.com/MrChromebox/coreboot.git
* cd coreboot
* git submodule update --init --checkout --recursive
* make crossgcc-i386
* ./build-uefi.sh <name of board to build>