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>

scripts/Makefile.lib

@@ -83,8 +83,8 @@ dtb-$(CONFIG_OF_ALL_DTBS)       += $(dtb-)
 multi-dtb-y := $(call multi-search, $(dtb-y), .dtb, -dtbs)
 # Primitive DTB compiled from *.dts
 real-dtb-y := $(call real-search, $(dtb-y), .dtb, -dtbs)
-# Base DTB that overlay is applied onto (each first word of $(*-dtbs) expansion)
-base-dtb-y := $(foreach m, $(multi-dtb-y), $(firstword $(call suffix-search, $m, .dtb, -dtbs)))
+# Base DTB that overlay is applied onto
+base-dtb-y := $(filter %.dtb, $(call real-search, $(multi-dtb-y), .dtb, -dtbs))
 
 always-y			+= $(dtb-y)