inittab: disable reboot by ctrl-alt-del

Most embedded systems do not have a keyboard.
If a keyboard is attached afterwards (e.g. to an accessible USB host port)
this allows the user to reboot the system by pressing Ctrl-Alt-Del, which
might not be desired.

Signed-off-by: lothar felten <felten@vaxnet.de>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>