kbuild: ignore make's built-in rules & variables

kbuild does explicitly specify what to do in all cases, and each
time make's built-in rules & variables has been used it has been a bug.
So to speed up things and to avoid the hard-to-debug error situations
ignore the built-in definitions.
If any part of the kernel uses the built-in definitions the build will
just stop there and it should be trivial to fix.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>