mkisofs needs to be installed first (use /usr/ports/sysutils/cdrtools).

Create a new directory to hold the contents of the CD-ROM in $CDROOT. Copy your kernel.gz and mfsroot.gz to it. Add the boot loader as follows:

mkdir $CDROOT/boot
cp $BOOTDIR/{cdboot,loader,loader.rc} $CDROOT/boot

Now use mkisofs to create the bootable ISO image:

mkisofs -b "boot/cdboot" -no-emul-boot -A "m0n0wall CD-ROM image" \
        -c "boot/boot.catalog" -d -r -publisher "foo.com" \
        -p "Your Name" -V "m0n0wall_cd" -o "m0n0wall.iso" \
        $CDROOT