core: do not accept multiple definitions of a package

One of the selling points for br2-external is to provide a mean to add
new packages. However, it is not supported that a package be defined by
Buildroot and then redefined in a br2-external tree.

This situation may occur without the user noticing or even willing to
redefine the package, for example:
  - br2-external is first created against a version of Buildroot
  - a package (missing in Buildroot) is added to that br2-external tree
  - upstream Buildroot adds this package
  - user updates to the new Buildroot

In this case, the result in undefined, and we can't make any guarantee
on the result (working or not).

Add a sanity check so that a package redefinition gets caught.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>