system: clarify issue/hostname settings

A custom skeleton may include /etc/issue and/or /etc/hostname, in
which case we do not want to override them.

But those files could also be missing from the custom skeleton, or
the user may want to override their content.

Update the documentation to make it clear that the user can set these to the
empty string to make buildroot not create/overwrite these files.

[Peter: drop the different default values depending on skeleton]
Signed-off-by: Sagaert Johan <sagaert.johan@skynet.be>
[yann.morin.1998@free.fr: change from hiding it behind SKELETON_DEFAULT
    to use conditional default values; move below the skeleton choice;
    improve help entries]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>