kbuild: determine base DTB by suffix

When using the -dtbs syntax, you need to list the base first, as
follows:

    foo-dtbs := foo_base.dtb foo_overlay1.dtbo foo_overlay2.dtbo
    dtb-y := foo.dtb

You cannot do this arrangement:

    foo-dtbs := foo_overlay1.dtbo foo_overlay2.dtbo foo_base.dtb

This restriction comes from $(firstword ...) in the current
implementation, but it is unneeded to rely on the order in the
-dtbs syntax.

Instead, you can simply determine the base by the suffix because
the base (*.dtb) and overlays (*.dtbo) use different suffixes.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>