buildroot

Config.in.legacy: update description for users

This patch clarifies the message shown to users in the legacy menu.
It explicitly mentions the need to save the configuration before disabling the
legacy options.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>