profiles: disable calling sync(2) after installing packages

This is just a safety feature for real Gentoo systems on the off chance
that they crash moments after a package is installed. There isn't any
such concern for the CoreOS SDK so just disable it. A normal build_image
is slightly faster with this but I suspect it may help the build host
which is usually running many emerge jobs in parallel.
updatectl: add ebuild for the CoreUpdate client
sdk-depends: add updatectl to SDK
Merge pull request #710 from marineam/updatectl

Add updatectl to SDK
coreos-kernel: enable dummy ethernet driver
coreos-kernel: prune old 3.14.x versions
systemd: remove timesyncd symlink from /etc

Along with the other services systemd is enabling in /etc the built in
SNTP client has been added to. Unlike the others we don't re-enable it
in /usr since we are currently using ntpd instead.
Merge pull request #713 from marineam/systemd

systemd: remove timesyncd symlink from /etc
Merge pull request #711 from marineam/dummy

Enable dummy ethernet driver
Merge pull request #709 from marineam/nosync

profiles: disable calling sync(2) after installing packages
Merge pull request #714 from crawford/cloudinit

coreos-base/coreos-cloudinit: bump to v0.8.2
Merge pull request #715 from crawford/master

coreos-base/coreos-cloudinit: bump to v0.8.3
coreos-base/coreos-cloudinit: bump to v0.8.5