dev-install: use profile stacking instead of copying files

Instead of duplicating /etc/portage/ into /usr/local/etc/portage/, use
the parent profile to let portage stack /usr/local/etc/portage/ on top
of /etc/portage/ dynamically.  This avoids confusing duplication and
takes care of the simple file that gmerge has been installing.

BUG=chromium:194889
TEST=precq passes & dev_install passes
CQ-DEPEND=CL:1497663

Change-Id: I54edf6142bb0a683837dbb65c63fb0b2abdef737
Reviewed-on: https://chromium-review.googlesource.com/1497668
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
1 file changed