[coreboot] initial Kconfig support for epia-n and via CPUs

[ron minnich]

This is not right, yet. But I could use a few comments.

one thing it does get right is the Makefile.inc for the CPUs, which is
done incorrectly in the tree (as is Intel, still, which I will try to
fix soon).

I have a question about the mainboard stuff. We want mainboard Kconfig
files in the mainboards, not in the next-level-up directory. But I
don't know how to get choice to work in the expected way. Please see
src/mainboard/via/Kconfig and
src/via/mainboard/*/Kconfig to see what I mean.

attached.

ron