Starting an initrd uImage on Le Potato

I am using Yocto Linux to generate an initrd uImate with a rescue system inside that is supposed to be booted in-memory. How can I configure the bootloader to boot into this image? I noticed my sd card partition has an extlinux.conf file with kernel command line arguments.
Should I modify that file to boot into an initrd image or is there some other change related to uboot I need to perform?