kbuild: get back DTB at the top directory

Commit 6ab6b2af deleted ./u-boot.dtb because it was a copy
of dts/dt.dtb.

But Simon suggested to keep u-boot.dtb at the top directory
because it is a build output.

After discussions, we agreed on revival of ./u-boot.dtb.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>