By default, syslinux interprets TFTP paths as relative to the location of the *.c32 modules, regardless whether a path starts with a slash. Without the `::` that I added to all of these paths, syslinux cannot find these files. It searches for them in /%INSTALL_DIR%/syslinux//%INSTALL_DIR%/boot/x86_64/vmlinuz-linux, etc.