imagine: you are using a device that requires
CONFIG_MYDEVICE_FROBOZZ_DRIVER but only if CONFIG_FROBOZZ has been
specified elsewhere. Because we check that every requested config
symbol actually appears in .config then it can't be added
unconditionally or the build will fail if CONFIG_FROBOZZ wasn't asked
for.
I'm not 100% happy about this design but it's the best I've thought of
so far.
we then "import" them straight back into base.nix - it's not
as though you can opt out of having a kernel. But this means
they'll appear separately in the documentation
