[U-Boot] Secure update of uboot devices?

With the new threats like Stuxnet and Duqu one need to improve on how new updates to software is distrubuted to devcies so one can be sure that it's not a hacked or modified software that runs in the device. We use uboot + usb stick to update the software in our devices. Have anyone been trying out to add e.g cryptated,certificates or rsa keys to uboot?