Further updates for syslinux v3:6.03~pre1+dfsg-4 support

We need to also include ldlinux.c32, libcom32.c32 and
libutil.c32, but only for syslinux versions >3:4.05+dfsg-6+deb8u1.

While the modules are located in directory
/usr/lib/syslinux/modules/bios/ the main isolinux.bin file is in
either /usr/lib/ISOLINUX or /usr/lib/syslinux, so adjust code
accordingly.