I was able to get fedora running using a kernel from a raspian image (as proposed here). But when I replace the kernel7.img with the vmlinuz-4.2.3-300.fc23.armv7hl from a Fedora-Minimal(armhfp) the multicolor bootloader screen does not disappear and the green led blinks twice after power on, then stays off.
On the vfat boot partition I have all the content of both images. I basically only replace kernel=vmlinuz-4.2.3-300.fc23.armv7hl in config.txt.
Where and why does my attempt fail?
Where and why does my attempt fail?
Your attempt fails because it is an attempt to use the wrong kernel. As mentioned in the Q&A you linked,
...the pi 2 still requires a special out-of-tree kernel...
Which, together with the firmware bootloader blob, is pretty much whole reason for needing to start with or pillage a Raspbian image (although as mentioned there you could also get that stuff from the upstream source).
The most upstream public source for the pi specific kernel is here. I don't think this contains further proprietary blobs, so why it hasn't been merged into the vanilla kernel is probably lack of motivation on behalf of the authors of the BCM specific stuff to jump through whatever hoops that would require.
If it does contain proprietary bits, then it is a licensing issue. In any case, the point is the vanilla linux kernel does not support the BCM 2835/6 SoC, hence there's a special kernel, and that's what you must use.
arch/arm/mach-bcm2708/9 is not in the vanilla kernel. The specific bits in arm/arch/ are often just tiny snippets of code specifying hardware addresses. I think multiple sets of these can be built in, producing a kernel which works on a variety of hardware (how the correct one gets selected I don't know). I'd guess without the correct one available it just fails immediately.