base-files: add support for restoring config from tmpfs

Instead of only relying in /sysupgrade.tgz being present in rootfs to
restore configuration, also grab /tmp/sysupgrade.tar which may have
magically gotten there during preinit...

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/base-files/files/lib/preinit/80_mount_root

@@ -17,11 +17,12 @@ missing_lines() {
 do_mount_root() {
 	mount_root
 	boot_run_hook preinit_mount_root
-	[ -f /sysupgrade.tgz ] && {
+	[ -f /sysupgrade.tgz -o -f /tmp/sysupgrade.tar ] && {
 		echo "- config restore -"
 		cp /etc/passwd /etc/group /etc/shadow /tmp
 		cd /
-		tar xzf /sysupgrade.tgz
+		[ -f /sysupgrade.tgz ] && tar xzf /sysupgrade.tgz
+		[ -f /tmp/sysupgrade.tar ] && tar xf /tmp/sysupgrade.tar
 		missing_lines /tmp/passwd /etc/passwd >> /etc/passwd
 		missing_lines /tmp/group /etc/group >> /etc/group
 		missing_lines /tmp/shadow /etc/shadow >> /etc/shadow