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.