Add custom first boot script

The first boot script is the one executed when the memory card is
freshly flashed and run on the board, this script is to initiate OS
environment. This custom script will genenate new secure keys in
/etc/ssh and back up to vendor partition using 'overlayroot' script.

Change-Id: Ibe84da3e33ffc893c319a9253793037c93a250fa

custom/templates/oem/overlay/root/firstboot.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+STAMP=/overlay/vendor/.stamp_sshkey
+
+PATH=/sbin:/usr/sbin:/bin:/usr/bin
+
+. /lib/init/vars.sh
+. /lib/lsb/init-functions
+
+if [ ! -f $STAMP ]; then
+	ssh-keygen -A
+
+	/usr/sbin/overlay-backup vendor
+
+	mount -o remount,rw /overlay/vendor
+	touch $STAMP
+	mount -o remount,ro /overlay/vendor
+fi
+
+exit 0