coreos-kernel: add experimental 3.18 kernel

For anyone who wants to test OverlayFS. To use add the following to
`/build/amd64-usr/etc/portage/package.keywords/kernel`:

    =sys-kernel/coreos-kernel-3.18.0 ~amd64
    =sys-kernel/coreos-sources-3.18.0 ~amd64
coreos-firmware: update to new snapshot, cleanup version detection

Now that the linux-info eclass works we can clean out some of the
version detection goo in coreos-firmware. Update snapshot for 3.18
sys-kernel: new kernel README file, everything has changed!
Merge pull request #999 from marineam/kernel

More kernel related updates