In order to create a UEFI payload for coreboot, I've started a coreboot platform as part of the TianoCore EDK II. The sources for the platform can be obtained here. Note that the CorebootPkg directory must be placed in the TianoCore $WORKSPACE directory.
To build the package on FreeBSD, a GNU toolchain from vendor sources must be used. This is because the TianoCore tools use some compiler/linker flags unknown to the toolchain included in the FreeBSD base system. The path as well as the names of the toolchain binaries must be adjusted in Conf/tools_def.txt. Because I built the toolchain according to these instructions, the preprocessor will not look in /usr/include for headers which causes errors in the ProcessorBind.h header when it attempts to include stdint.h. This patch can be applied to fix this.
Note that the build process still cannot complete as the tools producing the final Firmware Volume (FV) cannot cope with the ELF binaries produced by the GNU toolchain.