package/linux: fix custom dts files handling

package/linux: fix custom dts files handling

Custom dts files are still conditionally copied based on non existing
boolean. So it is currently not possible to use custom dts file(s) at all.

List of dts files is now iterated and files are copied into dedicated kernel arch dir.

Signed-off-by: Rafal Susz <rafal.susz@gmail.com>
Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>