fix(build_packages): Simplify emerge command, use board-packages ebuild.

The extra "dependency check" doesn't appear to be all that useful, so
don't waste time on it. Remove some unused command line options for
selecting which packages to build. Use new board-packages ebuild which
currently just includes 'coreos' and 'coreos-dev' but later can include
extra things like 'python-oem' which doesn't land in the base image.