rework emerge integration to run a single instance

The original design was that people could only install the latest version
of packages and that's it.  Requests are now coming in to support older
SLOTs (like getting python-2.7).  The current system turns the atoms into
make targets, but the full atom syntax includes bytes that are not valid
targets (such as the colon in "dev-lang/python:2.7").

Rework the modules so that we only run one instance of emerge, and give
it the full list of packages in one go.

TEST=`lunch brilloemulator_x86_64-eng && make -j32` now includes python in the image

Change-Id: Ia95e522572f4c944acbf449f93c702c6687f3fd1
1 file changed