Trouble building a working undionly.kpxe

[TheAmigo]

I'm doing my testing using VirtualBox 4.3.8 on Ubuntu 12.04. The reason I'm using undionly.kpxe is that I'm targeting a production environment that includes dozens of types of hardware and I want to use a single binary for everything.

When I download the pre-built undionly.kpxe (http://boot.ipxe.org/undionly.kpxe), it works fine.

But I want to add "console" and "colours" commands to it. So I downloaded the source via git and built my own undionly.kpxe.

With my build, I can download an initrd and a kernel, but when I boot the kernel, it can't find the initrd. I've been careful to change nothing in my setup other than undionly.kpxe. I can only assume I'm building it wrong.

Problematic builds start booting the kernel, but stop with "Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)". Good builds find the initrd, get past that and boot the OS without further issue. Currently, I can build versions that fall into one of three categories:
1) can't load a background picture, but can boot an OS
2) can load a background picture, but can't boot an OS
3) can't do either (obviously... we can all make something that doesn't work )

I have good builds that are larger than bad builds so I'm not suspecting size... or should I? Is there a size limit for virtualbox?

Suggestions welcome.