system: add help entry to "none" init system

It can be a little bit misleading to have no init system...

Add a comment that states the user has to provide his own init system,
either via a package or a rootfs overlay.

It is expected that such a user will know what to provide, so we don't
really need to specify that it should be /init or /sbin/init or any
arbitrary executable pointed to by the kernel command line "init=..."
or anything else...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>