Add a /etc/rc.local in the common rootfs-overlay.

rootfs-overlay/common/etc/rc.local

@@ -0,0 +1,17 @@
+#!/bin/sh -e
+#
+# rc.local
+#
+# This script is executed at the end of each multiuser runlevel.
+# Make sure that the script will "exit 0" on success or any other
+# value on error.
+#
+# In order to enable or disable this script just change the execution
+# bits.
+#
+# By default this script does nothing.
+
+# Regenerate SSH keys on first boot.
+[ -f /etc/ssh/ssh_host_rsa_key.pub ] || ssh-keygen -A
+
+exit 0