user.eclass: default to SYSROOT rather than ROOT for repo details

We already rely on SYSROOT for PORTAGE_CONFIGROOT.  More importantly,
newer portage isn't able to find details under $ROOT that we need.
Lets get the tree green again while we figure out the next step.

BUG=chromium:1026502
TEST=CQ passes

Change-Id: If7dacb644d3eaea9af06c5b9e110b8b6fa2cc71a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/1925714
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/eclass/user.eclass b/eclass/user.eclass
index eb5b3df..5ac4661 100644
--- a/eclass/user.eclass
+++ b/eclass/user.eclass
@@ -77,8 +77,9 @@
 	fi
 
 	local overlay
-	for overlay in $(_call_portageq get_repos "${ROOT:-/}") ; do
-		local overlay_dir=$(_call_portageq get_repo_path "${ROOT:-/}" "${overlay}")
+	local root="${SYSROOT:-${ROOT:-/}}"
+	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."