Have lcrs_embedded installed in Buildroot

[benjaoming]

In LCRS2, we ran our own script after building the output. It is run before building the actual images.

The script did this:
https://github.com/fairdk/lcrs2/blob/master/buildroot/postbuild.sh

Perhaps this is still the best and most flexible option. But it should ideally invoke python setup.py sdist to pack an actual distributed version of lcrs-embedded and then pip install it into a complete virtual environment of the host target with all dependencies?

Another option would be if there's somehow already a recipe for installing custom python packages and dependencies on a target system.

[benjaoming]

I have a feeling that redistributable virtualenvs aren't the way to go, because virtualenv --relocatable <dir> needs to be run inside the target system - and because it's still considered buggy and experimental (see: https://github.com/pypa/virtualenv/issues/11)