Looking at the "1.8b Development" board on the forums, I came across
this post : http://forum.m0n0.ch/index.php/topic,6065.0.html
A moderator stated that this was a bug, I looked at the
webgui/system_firmware.php file in order to propose a fix. You'll find
it in attachment.
I also tried to clarify the directions given at the top of the page.