""" However, sometimes it's nice to be able to update the kernel all by itself, or leave a kernel update out of your regular whole system update. To do this, we will create a new package set called "kernel". """
This means two things:
1) We can update the kernel without updating the rest of the system by issuing `emerge -auDN --with-bdeps=y @kernel`. It works.
2) We can update the whole system without updating the kernel by issuing `emerge -auDN --with-bdeps=y @world`. This doesn't work!
Here is the output of `emerge -tpvuDN --with-bdeps=y @world`: https://bpaste.net/show/221b8e102634
As you can see, portage wants to update sys-kernel/vanilla-sources, though not in world.