core: make NotifyAccess= in combination with RootDirectory=/RootImage= work

core: make NotifyAccess= in combination with RootDirectory=/RootImage= work

Previously if people enabled RootDirectory=/RootImage= and NotifyAccess=
together, things wouldn't work, they'd have to explicitly add
BindReadOnlyPaths=/run/systemd/notify too.

Let's make this implicit. Since both options are opt-in, if people use
them together it would be pointless not also defining the
BindReadOnlyPaths= entry, in which case we can just do it automatically.

See: #18051