pkg-infra: introduce errors for legacy API

As discussed in the BR developer days, we want to be more strict about API
changes in buildroot. I.e., we want to make it less likely that a user's
customizations break down after upgrading buildroot.

A first step is to make sure that the user is warned about API changes.
This patch introduces Makefile.legacy and Config.in.legacy, which will
issue clear error messages for such situations.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>