Makes some changes in order to properly generate VMware OEM images

Defines oem-vmware package so it is installed and built along with its dependencies when generating VMware images.

This PR is part of a larger change-set intended to create a good starting point for VMware OEM images. The change-set is composed of the following PRs: https://github.com/coreos/scripts/pull/336, https://github.com/coreos/coreos-overlay/pull/898, https://github.com/coreos/init/pull/137, https://github.com/coreos/portage-stable/pull/139