fix(virtual/linux-sources): Only install coreos-kernel on targets.

We don't have a valid kernel (or use-case to have one) for "cros_host"
(the SDK) so just fake it. Also remove some unused flags.

This change prevents the latest kmod ebuild from pulling in
coreos-kernel, bootengine, and friends into the SDK.
Merge pull request #155 from marineam/systemd

fix(virtual/linux-sources): Only install coreos-kernel on targets.