This is not correct : make crossgcc && make iasl && CPUS=4
This is correct : make crossgcc CPUS=4 && make iaslThis will build the entire toolchain, but you don't need the whole thing. (take a lot of time to compile as well)
If you want to build only x86 & x64 you use (from the coreboot folder) : make crossgcc-i386 CPUS=4 && make crossgcc-x64 CPUS=4 && make iasl then : make menuconfig To configure the build options for the rom.
