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.
Slightly simplifies adding boot loader configuration for more kernels.
Unfortunately the INCLUDE statement doesn't support wildcards, so each new file must be manually included in the main syslinux configuration file (syslinux.cfg for baseline and archiso_sys.cfg for releng).
