Custom Initramfs image with nfs support

Hello all,

I'm using a custom initramfs image as this is the most efficient
solution for my setup. But by using a custom initramfs image I lose the
original initramfs code that is included in the default initramfs image.

Normally when using the default initramfs image I can use the
root=/dev/nfs kernel parameter to use a nfs share as root file system.
When I'm using my own initramfs image there is no code included for
mounting a root file system and so it doesn't work.

How can I have the same functionality like in the default initramfs
image? Where can I find what is included in the initramfs image by
default? And how can I include this in my own image?
My root file system used for the initramfs image is created with
buildroot. Busybox is included, and is used as the init program.

Thanks in advance,
Dries