builddeb: Fix inclusion of dtbs in debian package

Commit 37c8a5fa ("kbuild: consolidate Devicetree dtb build rules")
moved the location of 'dtbs_install' target which caused dtbs to not be
installed when building debian package with 'bindeb-pkg' target. Update
the builddeb script to use the same logic that determines if there's a
'dtbs_install' target which is presence of the arch dts directory. Also,
use CONFIG_OF_EARLY_FLATTREE instead of CONFIG_OF as that's a better
indication of whether we are building dtbs.

This commit will also have the side effect of installing dtbs on any
arch that has dts files. Previously, it was dependent on whether the
arch defined 'dtbs_install'.

Fixes: 37c8a5fa ("kbuild: consolidate Devicetree dtb build rules")
Reported-by: Nuno Gonçalves <nunojpg@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>