query ROOT for the active overlay list

The SYSROOT is for use while building (src_*) while the ROOT is for use
when merging packages (pkg_*).  Have the user logic utilize ROOT to find
the overlay logic so that a single binary package built against a SYSROOT
can be merged against multiple ROOTs.

Otherwise, when we try sharing binpkgs (like stumpy with stumpy_moblab),
the package will remember the original SYSROOT and try to re-use that.
But there's no guarantee that the SYSROOT still exists.

BUG=chromium:370487
TEST=`cbuildbot stumpy_moblab-release sandybridge-ivybridge-full-group` passes

Change-Id: Id69cecd60f597b568238766c74dfd502244120c2
Previous-Reviewed-on: https://chromium-review.googlesource.com/200252
(cherry picked from commit 59752a96e10313d3e6b540a5cb77881fb30cb9a3)
Reviewed-on: https://chromium-review.googlesource.com/203000
Reviewed-by: Chris Masone <cmasone@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/eclass/user.eclass b/eclass/user.eclass
index 1df3966..12e1b20 100644
--- a/eclass/user.eclass
+++ b/eclass/user.eclass
@@ -66,8 +66,8 @@
 	fi
 
 	local overlay
-	for overlay in $(_call_portageq get_repos "${SYSROOT:-/}") ; do
-		local overlay_dir=$(_call_portageq get_repo_path "${SYSROOT:-/}" "${overlay}")
+	for overlay in $(_call_portageq get_repos "${ROOT:-/}") ; do
+		local overlay_dir=$(_call_portageq get_repo_path "${ROOT:-/}" "${overlay}")
 		local accounts_dir="${overlay_dir}/profiles/base/accounts"
 		if [[ -d "${accounts_dir}" ]] ; then
 			einfo "Adding ${accounts_dir} to user/group search path."