Add an element to configure a serial console.

With ILO and other remote machine management cards, a serial console is
actually useful.

Change-Id: I86f74d9dae3821b4682df813c5b52a83a29a138a

elements/serial-console/README.md

@@ -0,0 +1,5 @@
+Start getty on ttyS0 or ttyS1
+
+With ILO and other remote admin environments, having a serial console can be
+useful for debugging / troubleshooting. If ttyS1 exists, getty will run on
+that, otherwise on ttyS0.

elements/serial-console/install.d/20-stty

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+#!/bin/sh
+
+home=$(dirname $0)
+set -uex
+
+install -m 0644 -o root -g root $home/ttySx.conf /etc/init/ttySx.conf
+

elements/serial-console/install.d/ttySx.conf

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+# This service maintains a getty on ttyS0/1 from the point the system is
+# started until it is shut down again.
+
+start on stopped rc or RUNLEVEL=[2345]
+stop on runlevel [!2345]
+
+respawn
+
+script
+        console_port=0
+        echo "ttySx probing ttyS1" >/dev/ttyS1 2>/dev/null && console_port=1
+        echo "ttySx:  console tty = /dev/ttyS$console_port" >&2
+        exec /sbin/agetty -m ttyS$console_port 115200,57600,38400,9600,4800 vt102
+
+end script