Config.in: Add a menu around BR2_EXTERNAL configuration options

To make it clear to the user that these options are external to Buildroot
and no support is provided by the Buildroot community.

With this, there's no need to recommend people to their options inside
a menu, so remove that from the documentation.

Kconfig nicely allows us to hide the menu completely if BR2_EXTERNAL isn't
used, so make use of that to not confuse people. It would be nice if we
could add some help text to explain the BR2_EXTERNAL stuff as well, but
that isn't supported on menus :/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>