Add support to update cache of overlays

etc/portage/postsync.d/sync_cache

@@ -4,7 +4,20 @@ source /etc/portage/check-portdir.sh
 source /etc/init.d/functions.sh
 source /etc/portage/util-functions.sh
 
-ebegin "Syncing metadata cache"
+# add your overlays here (name of the overlay!)
+overlay_list=(
+)
+
+ebegin "Syncing metadata cache for gentoo repository"
 rsync -avPq rsync://rsync.gentoo.org/gentoo-portage/metadata/md5-cache/ "${PORTDIR}"/metadata/md5-cache/
 egencache --jobs=$(nproc) --repo=gentoo --update --update-use-local-desc
 eend $?
+
+if [[ ${#overlay_list[@]} -gt 0 ]] ; then
+	ebegin "Syncing metadata cache for overlays"
+	for _overlay in ${overlay_list[@]} ; do
+		einfo "  ${_overlay}"
+		egencache --jobs=$(nproc) --repo=${_overlay} --update --update-use-local-desc
+	done
+	eend $?
+fi