linux-dtb.inc: drop unused DTB_NAME variable from do_install

* this is causing do_install to depend on KERNEL_IMAGE_BASE_NAME which
  in some cases contains something like BUILD_NUMBER from CI, that
  caused do_install to be reexecuted every single time, which is very
  sad to be caused by unused variable.
* jethro and newer don't need this change, because it's also fixed in
  commit 86b3f29f
  Author: Marek Vasut <marex@denx.de>
  Date:   Thu May 14 14:31:11 2015 +0200
  Subject: kernel: Build DTBs early

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joshua Lock <joshua.lock@collabora.co.uk>