fix(make.conf): Don't share binary packages between boards.

Although it'd be nice to re-use packages for experimental boards that
may only have a few differences from amd64-generic there is a bug in
some ebuilds and portage that break sharing binary packages between
different values of $ROOT. This prevents that from happening by
accident.

https://bugs.gentoo.org/show_bug.cgi?id=490014
Merge pull request #294 from marineam/switch-usr

fix(make.conf): Don't share binary packages between boards.