recipes-bsp: Add support for gpio-shutdown

- Feature is enabled with ENABLE_GPIO_SHUTDOWN variable
- Include the gpio-shutdown overlay
- If using SysV init include the additional keymap and inittab entries
  to bind the KEY_POWER event
- Systemd init does not require any additional bindings

Signed-off-by: Otto Esko <otto.esko@gmail.com>

recipes-bsp/gpio-shutdown/files/bind_gpio_shutdown.tab

@@ -0,0 +1,2 @@
+# Action on special keypress (Key Power)
+kb::kbrequest:/sbin/shutdown -t1 -a -h -P now

recipes-bsp/gpio-shutdown/files/gpio-shutdown-keymap.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+##
+# Bind the gpio-shutdown keycode as Keyboard signal and load it to the
+# keymap during startup.
+##
+case "$1" in
+    start)
+    # Inject the gpio keycode to keymap
+    echo "keycode 116 = KeyboardSignal" | loadkeys
+    ;;
+    *)
+    ;;
+esac