infra: replace BUILDROOT_CONFIG with BR2_CONFIG

To make the naming consistent (all user-visible options should be
prefixed with BR2_).

An entry is added to Makefile.legacy to warn users who have set
BUILDROOT_CONFIG but not BR2_CONFIG.

Still export BUILDROOT_CONFIG but pointing to some phony value, to
make sure that scripts that still use it fail in a predictable way.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>