factory: add default /etc/issue file

Booting up an image with --volatile=yes otherwise looks so naked, so
let's include this file in the default factory too. It's common and
simple and should be safe to ship.

factory/etc/issue

@@ -0,0 +1,3 @@
+\S
+Kernel \r on an \m (\l)
+

tmpfiles.d/etc.conf.m4

@@ -20,3 +20,4 @@ C /etc/nsswitch.conf - - - -
 m4_ifdef(`HAVE_PAM',
 C /etc/pam.d - - - -
 )m4_dnl
+C /etc/issue - - - -