buildroot

Remove the BOARD/LOCAL feature

This can be achieved by adding a new target in target/device/, and I
will later propose a mechanism to add external boards and packages, so
that our users not willing to get their target-specific or
package-specific things merged can keep them cleanly separated from
Buildroot tree.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>