base-files: add simple shutdown script

Since svn rev [38557] we support power buttons, for example
the ACPI power button on x86 machines.
Add a simple shutdown script to /etc/rc.button, to gracefully shutdown
the machine after the power button is pressed.

Tested on x86 and x86_64 in a kvm virtual machine.

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>

SVN-Revision: 40909

package/base-files/files/etc/rc.button/power

+#!/bin/sh
+
+[ "${ACTION}" = "released" ] || exit 0
+
+exec /sbin/poweroff