Add post update script and service

etc/systemd/system/post-update.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+if [ ! -f /post-update-done ]; then
+    steamos-readonly disable
+    pacman-key --init
+    pacman-key --populate archlinux
+    pacman -S --noconfirm glibc
+    sed -i 's/#\(ja_JP.UTF-8.*\)/\1/' /etc/locale.gen
+    locale-gen
+    touch /post-update-done
+    steamos-readonly enable
+fi