I think the first step would be figuring out how to get a kernel onto the device; this is an area I've never delved into, but I read about it and it's really in the realm of electronic engineers (JTAG port this, oscilloscope that, etc. - I don't mess with this stuff) whereby they somehow figure out how to shove an image onto the device. One of the first things that comes to mind is the DD-WRT type projects, they had to somehow use these lowlevel hardware interface methods to discover how they could get something to boot.