arch/x86/boot/tools/build is a tool to buld a disk-image (like arch/x86/boot/bzImage) from the following three files: setup.bin (8068 machine code to set up system parm)
vmlinux.bin (80386 code for actual systems)
zoffset.h (Header with ZO_* defines) arch/x86/boot/tools/build is to update some pre-reserved fields that are declared in arch/x86/boot/header.S (such as the size of the resulting bzImage, the address of the entry point). arch/x86/boot/tools/build.c