add init script when overclocking that allows the user to opt out on boot

Current kernel builds default to the powersave cpufreq governor, with the
intention that this is changed to 'ondemand' after boot. When the user sets
overclock in raspi-config, an init script is installed which does this switch,
but only does it if it doesn't detect a shift key is held down. This means the
user can still boot the system if they set an overclock setting that is higher
than their device can manage.

raspi-config

@@ -220,6 +220,43 @@ set_overclock() {
   set_config_var core_freq $3 /boot/config.txt &&
   set_config_var sdram_freq $4 /boot/config.txt &&
   set_config_var over_voltage $5 /boot/config.txt &&
+  # now set up an init.d script
+cat <<\EOF > /etc/init.d/switch_cpu_governor &&
+#!/bin/sh
+### BEGIN INIT INFO
+# Provides:          switch_cpu_governor
+# Required-Start: udev mountkernfs $remote_fs
+# Required-Stop:
+# Default-Start: S
+# Default-Stop:
+# Short-Description: Switch to ondemand cpu governor (unless shift key is pressed)
+# Description:
+### END INIT INFO
+
+. /lib/lsb/init-functions
+
+case "$1" in
+  start)
+    log_daemon_msg "Checking if shift key is held down"
+    timeout 1 thd --dump /dev/input/event* | grep -q "LEFTSHIFT\|RIGHTSHIFT"
+    if [ $? -eq 0 ]; then
+      printf " Yes. Not switching scaling governor"
+      log_end_msg 0
+    else
+      SYS_CPUFREQ_GOVERNOR=/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
+      [ -e $SYS_CPUFREQ_GOVERNOR ] && echo "ondemand" > $SYS_CPUFREQ_GOVERNOR
+      printf " No. Switching to ondemand scaling governor"
+      log_end_msg 0
+    fi
+    ;;
+  *)
+    echo "Usage: $0 start" >&2
+    exit 3
+    ;;
+esac
+EOF
+  chmod +x /etc/init.d/switch_cpu_governor &&
+  update-rc.d switch_cpu_governor defaults &&
   whiptail --msgbox "Set overclock to preset '$1'" 20 60 2
 }