pkg-generic: introduce helper function to check for a deprecated variable

This patch introduces a helper check-deprecated-variable, that throws an
error when a package defines a deprecated variable. This helps in detecting
transition errors when changing the buildroot infrastructures, and also
helps users that have created packages not part of mainline buildroot.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>