Add a default message of the day (/etc/motd)

This is injected to the factory-config during bootstrap, and can then
be changed as usual via /system/motd.

board/common/rootfs/etc/motd

@@ -0,0 +1,4 @@
+.-------.
+|  . .  | Infix -- a Network Operating System
+|-. v .-| https://kernelkit.github.io
+'-'---'-'

src/confd/bin/Makefile.am

@@ -1,2 +1,2 @@
 pkglibexec_SCRIPTS  = bootstrap error load \
-		      gen-hostkeys gen-admin-auth gen-hostname gen-interfaces
+		      gen-hostkeys gen-admin-auth gen-hostname gen-interfaces gen-motd

src/confd/bin/bootstrap

@@ -97,6 +97,7 @@ factory()
 
     # Create an overlay for /etc/hostname to change the default in an br2-external
     gen-hostname                                            >"$FACTORY_D/20-hostname.json"
+    gen-motd                                                >"$FACTORY_D/20-motd.json"
     # shellcheck disable=SC2086
     gen-interfaces $GEN_IFACE_OPTS                          >"$FACTORY_D/20-interfaces.json"
 

src/confd/bin/gen-motd

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+set -e
+
+if [ -f /etc/motd ]; then
+    cat <<EOF
+{
+  "ietf-system:system": {
+    "infix-system:motd": "$(sed ':a;N;$!ba;s/\n/\\n/g;s/\t/\\t/g' /etc/motd)"
+  }
+}
+EOF
+else
+    echo "{}"
+fi