syslinux package got split-up in Debian/jessie

pxelinux.0 is in ```pxelinux``` now, menu.c32 and chain.c32 are in ```syslinux-common``` now:
https://packages.debian.org/jessie/amd64/pxelinux/filelist
https://packages.debian.org/jessie/amd64/syslinux-common/filelist