I've read through the U-Boot manual and FAQ, but I still haven't figured out how one downloads via the network an image to a board running U-Boot. Previous projects have used RedBoot, and it provided a couple different mechanisms:
1) Telnet to the board and download an image (binary or S-Record) using the x/y-modem protocol over the telnet connection.
2) Download a binary image directly using a TCP connection to a dedicated port.
Both of these methods would work through firewalls and WAN connections (even through satellite links), and could easily be automated in an "updater" utility that is then provided to customers to update images in flash.
I can't seem to find out how one accomplishes the same task using U-Boot. The only method I can figure out involve setting up a TFTP server (which is not going to be acceptible to customers), and then typing a series of commands while plugging into a serial console (also not going to be acceptible to customers). The requirement is to update the image using just the network and a user-space application that doesn't require any system privledges to run.
I found mention of netconsole, but I don't see how it's useful since you have to know a-priori the address of the machine from which you want to use it. It would seem that you have to force the customer to change the IP address of their machine (not acceptible).