portage: Add patch to filter EROOT from ebuild environment

EROOT must always be derived from ROOT, preserving it causes binary
packages to break in mysterious and often silent ways.

Upstream: https://bugs.gentoo.org/show_bug.cgi?id=490014
Related: https://github.com/coreos/portage-stable/pull/125

This change doesn't impact images just yet since this portage version is still marked unstable. The old binutils will need to be flushed from our system before we can safely use portage with this patch.