I recently tried to enable hardware VT extensions on my Chromebook using the Hacking VMX Support instructions on the ChromiumOS website and it didn’t work. Not only did it not work, I also had a second go at doing it, following the instructions verbatim (despite it leaving 3 “bad” instructions in there). This left me with an expensive paper-weight. This is how I’ve come to know how to unbrick the 550.
If you are here I’m assuming you are in a similar situation, and, hopefully, have a copy of the original firmware to hand, with which to unbrick. You will also need another laptop with a suitable Linux distro loaded, such as Ubuntu/Fedora, and have Flashrom installed. Onto parts. You will need a Bus Pirate, a Bus Pirate probe cable, and a Pomona 5250 8 pin SOIC clip. Don’t buy a 3M SOIC clip, the spring is far too strong, and it’s designed in such a way that the pins won’t make contact unless you “modify” it, see http://www.eevblog.com/forum/microcontrollers/3m-soic8-clip-waste-of-timemoney/msg33462/?PHPSESSID=e4d9fa7c85d3aee85d389405447973cc#msg33462