setup_board: copy portage's repos.conf files from the chroot
tools: remove support for parallel_emerge

Using parallel_emerge has been disabled by default for all commands
except build_image for quite a while now, build_image kept it just
because it was still a bit faster than normal emerge. Keeping
parallel_emerge complicates future changes to build_image so it needs to
drop it entirely. Since that means nothing uses it by default we might
as well just rip out support for it entirely.
build_image: use dev and prod profile variants

Instead of gluing in a special PROD_INSTALL_MASK for all images use
profiles to configure the differences between the base build root,
production images, and developer images. This offers much more
flexibility and is needed for providing a full dev environment in
developer images.
build_image: install the complete system package set in dev images

dev images, now with fancy tools like `make`!
build_image: add repos.conf and binpkgs url to dev images

Needed for portage 2.2. Sync URIs are included but not very useful yet
because portage only can do `git pull` but not `git clone`. An extra
helper script will be required to do the initial clone it seems.

Binary packages may be useful for re-installing a package with a
different INSTALL_MASK. Can be used to install debug symbols.
Merge pull request #302 from marineam/dev

Developing in dev images