fix(coreos-base/*): Replace sys-fs/udev with virtual/udev

In preparation for systemd 204 which provides udev itself we need to
stop depending directly on the stand-alone udev package.
fix(coreos-base/coreos-base): Install systemd in SDK

Adding systemd to base means it will be installed in the SDK in addition
to built images. This should avoid potential confusion that may be
caused by leaving sys-fs/udev in the SDK while it is removed from built
images in favor of systemd 204.
hack(sys-apps/openrc): Mark as provided in the SDK

We don't need openrc in the SDK either so move the package.provided
entry to the default profile instead of just the coreos target.
bump(sys-apps/systemd): unmask systemd-204

Booted! Ship it!
Merge pull request #6 from marineam/upgrade-systemd

Upgrade systemd