fix(vm_image_util): Keep most partitions mounted read-only.

This ensures that the only partition being modified is OEM.
fix(vm_image_util): Require OEM packages to be aware of /usr/share/oem

Installing to a temporary directory and then copying over the final
contents of /usr/share/oem allows more complicated OEM packages such as
python to be configured with --prefix=/usr/share/oem while previously
the atypical use of ROOT=/usr/share/oem would have complicated things.
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.
Merge pull request #208 from marineam/oem

Rework how OEM packages are installed