I know of *one* eReader based on a semi-recent Yocto + NXP image: that's the reMarkable, and it runs on Linux 4.9 (which is what the official NXP+EPDC tree jumped to after the 4.1 used on Kobo).
That may be worth looking into, as the SDK is pretty much what Yocto's buildsystem outputs, IIRC (I'm, err, not necessarily a fan of Yocto, then again, not particularly familiar with it either, as I've never really had cause to use it
).
(Also, the rM's app also runs on Qt5 like Kobo's Nickel).
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TL;DR: Getting the userland up-to-date should be "trivial". The kernel, eeeh, might be trickier. Especially since Kobo themselves apparently barely touch it, AFAICT, that's left entirely at the mercy of NTX, their OEM.