Add repo_name and layout.conf for portage-stable.

The layout.conf fails declares portage, this overlay and chromiumos as the
reverse-ordered list of overlays to search for eclasses.

Change-Id: I5506c0ca03e9e7d0891f7c08f5463ba9512f74f7

BUG=none
TEST=./setup_board && ./build_packags for {x86,arm}-generic

Review URL: http://codereview.chromium.org/6758048