deb-pkg: Add device tree blobs to the package

When building a package with make deb-pkg (say, for arm), the dtb files are
not added to the package. Given that things are still evolving on arm, it
make sense to have them along with the kernel and modules.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Reviewed-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: maximilian attems <max@stro.at>
Signed-off-by: Michal Marek <mmarek@suse.cz>