kbuild: create a list of all built DTB files

It is useful to have a list of all *.dtb and *.dtbo files generated
from the current build.

With this commit, 'make dtbs' creates arch/*/boot/dts/dtbs-list, which
lists the dtb(o) files created in the current build. It maintains the
order of the dtb-y additions in Makefiles although the order is not
important for DTBs. It is a (good) side effect through the reuse of the
modules.order rule.

Please note this list only includes the files directly added to dtb-y.

For example, consider this case:

    foo-dtbs := foo_base.dtb foo_overlay.dtbo
    dtb-y := foo.dtb

In this example, the list will include foo.dtb, but not foo_base.dtb
or foo_overlay.dtbo.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>