Funny coincidence, I'm developing okreader (https://github.com/lgeek/okreader), which is a package of u-boot, deblobbified downstream kernel images, Debian and KOReader (https://github.com/koreader/koreader) for a range of Kobo devices. I've been working with upstream KOReader developers to get the features required to use KOReader without the Kobo firmware. okreader replaces the whole software stack on the device.