use overlayfs for /etc

recipes-core/base-files/base-files_%.bbappend

@@ -1,7 +1,7 @@
 hostname = "diya"
 do_install:append () {
     cat >> ${D}${sysconfdir}/fstab <<EOF
-# add bind mount for /etc
-/etc /var/etc none defaults,bind 0 0
+# add overlay for /etc
+overlay /etc overlay defaults,nofail,lowerdir=/etc,upperdir=/var/etc/upper,workdir=/var/etc/work 0 2
 EOF
 }