How to fix boot into initramfs prompt and “mount: can’t read ‘/etc/fstab’: No such file or directory” and “No init found”?

Installing a new system using a GPT partitioned disk dedicated to a single partition, ext4 formatted, extlinux (version 4.05) as bootloader, Ubuntu Core version 13.10 amd64 as rootfs, and Ubuntu linux-image-3.11.0-18-generic as kernel, and extlinux-update to generate bootloader configuration.

How to mount multiple directories on the same partition?

I am installing an SSD and would like to put / on the SSD and /home, /var, and /tmp on the HDD. My current distro is Kubuntu but I would not mind trying another distro if this procedure can be accomplished easier there. I have installed many different Linux OSes on multiple partitions, however I know of no installer that lets one mount multiple directories on a single partition. I would rather not use three separate partitions as particularly /home, /var, and /tmp are prone to large changes in size and it is not practical to allot each of them some arbitrary maximum.