metag: use common make variables for dtb builds

Use dtb-y and always make variables to build dtbs instead of explicit
dtbs rule. This is in preparation to support building all dtbs.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org