fix(coreos-base/coreos-dev): Add coreos and gcc as dependencies.

This makes coreos-dev a proper superset of coreos.
add(coreos-devel/board-packages): Target for building binary packages
fix(coreos-base/oem-*): Install OEM packages into /usr/share/oem

The install script no longer sets ROOT to /usr/share/oem so the ebuilds
need to install themselves into the correct path.
Merge pull request #494 from marineam/oem

OEM reorg