sys-kernel/coreos-sources: fetch kernel sources via tarballs

Providing the kernel source via git is overkill, we do not maintain any
patches on top of the mainline kernel any more and the repository is
huge. As an added benefit this installs the kernel source to the
traditional /usr/src/linux location that Gentoo expects. Hopefully that
will reduce the number of warnings many ebuilds tend to output.
coreos-kernel: new kernel build based on coreos-sources

This cuts out the need for git for our kernel sources and the + and the
end of our versions is now gone. The build remains a bit weird in order
to bundle the initrd into the kernel image but eventually that will be
dropped, leaving the new coreos-kernel eclass pretty tiny.

Bonus: That + at the end of our versions is finally gone!
Merge pull request #996 from marineam/kernel

Build kernels from tarballs