It's been a while since I last worked on combining TianoCore and coreboot. Tonight I had some spare time and tried to pursue the project.
The previously mentioned build failure does indeed stem from the fact that the build tools cannot cope with ELF binaries. Especially problematic is the GenFw tool which is supposed to convert the binary file into an UEFI firmware volume file. In order to do that, it parses the header information of the input binary executable file and encodes the type of file (in UEFI terms) in a spare header field. The tool expects to work on PE32 files but the TianoCore developers have added code which converts an ELF image into a PE32 image internally if the tool is pointed at an ELF file. However, this facility is only compiled in if #defined(Linux) is true. Of course, that won't work on FreeBSD but changing the relevant pre-processor condition allowed me to produce an UEFI firmware volume without any further changes to the code.
However, this shortcut will only work on x86 and only if the target platform is x86, too. The real solution is to avoid the conversion and instead encode the UEFI file type directly into the ELF header. I've already done this for my thesis project (*) and back then it seemed that re-writing the GenFw tool was easier than fixing the existing implementation. Well, here's the next item on the ToDo list...
(*) I used the Java-based tools for the thesis project which means that a different tool with essentially the same functionality was the culprit.