define initramfs scripts for ubuntu

provide a hook to require overlayfs module so the file system is available
define a general script to overlay any readonly root filesystem into a non-persistent, writable filesystem

initramfs/ubuntu/hooks/grub-btrfs-overlay

@@ -0,0 +1,14 @@
+#!/bin/sh -e
+PREREQ=
+prereqs() {
+	echo "$PREREQ"
+}
+case "$1" in
+	prereqs)
+		prereqs
+		exit 0
+		;;
+esac
+
+. /usr/share/initramfs-tools/hook-functions
+manual_add_modules overlay