Adjust syslinux directories for syslinux v3:6.03~pre1+dfsg-4

Recent versions of syslinux no longer provide the files we have
been relying on in the syslinux package any longer but files have
been split into packages isolinux, pxelinux, syslinux + syslinux-common
with different directory names.
SW: move syslinux into GRMLBASE class, add pxelinux + isolinux

Recent versions of syslinux (at least applies to v3:6.03~pre1+dfsg-4)
split the packages into isolinux, pxelinux, syslinux + syslinux-common.