kbuild: fix compilation of dtb specified on command-line without make rule

Since commit e7e2941 ("kbuild: split device tree build rules into
scripts/Makefile.dtbs"), it is no longer possible to compile a device tree
blob that is not specified in a make rule
like:
    dtb-$(CONFIG_FOO) += foo.dtb

Before the mentioned commit, one could copy a dts file to e.g.
arch/arm64/boot/dts/ (or a new subdirectory) and then convert it to a dtb
file using:
    make ARCH=arm64 foo.dtb

In this scenario, both 'dtb-y' and 'dtb-' are empty, and the inclusion of
scripts/Makefile.dtbs relies on 'targets' to contain the MAKECMDGOALS. The
value of 'targets', however, is only final later in the code.

Move the conditional include of scripts/Makefile.dtbs down to where the
value of 'targets' is final. Since Makefile.dtbs updates 'always-y' which is
used as a prerequisite in the build rule, the build rule also needs to move
down.

Fixes: e7e2941 ("kbuild: split device tree build rules into scripts/Makefile.dtbs")
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20251126100017.1162330-1-thomas.de_schampheleire@nokia.com
Signed-off-by: Nicolas Schier <nsc@kernel.org>

Author: patrickdepinguin

scripts/Makefile.build

@@ -527,18 +527,6 @@ ifneq ($(userprogs),)
 include $(srctree)/scripts/Makefile.userprogs
 endif
 
-ifneq ($(need-dtbslist)$(dtb-y)$(dtb-)$(filter %.dtb %.dtb.o %.dtbo.o,$(targets)),)
-include $(srctree)/scripts/Makefile.dtbs
-endif
-
-# Build
-# ---------------------------------------------------------------------------
-
-$(obj)/: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
-	 $(if $(KBUILD_MODULES), $(targets-for-modules)) \
-	 $(subdir-ym) $(always-y)
-	@:
-
 # Single targets
 # ---------------------------------------------------------------------------
 
@@ -568,6 +556,20 @@ FORCE:
 targets += $(filter-out $(single-subdir-goals), $(MAKECMDGOALS))
 targets := $(filter-out $(PHONY), $(targets))
 
+# Now that targets is fully known, include dtb rules if needed
+ifneq ($(need-dtbslist)$(dtb-y)$(dtb-)$(filter %.dtb %.dtb.o %.dtbo.o,$(targets)),)
+include $(srctree)/scripts/Makefile.dtbs
+endif
+
+# Build
+# Needs to be after the include of Makefile.dtbs, which updates always-y
+# ---------------------------------------------------------------------------
+
+$(obj)/: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
+	 $(if $(KBUILD_MODULES), $(targets-for-modules)) \
+	 $(subdir-ym) $(always-y)
+	@:
+
 # Read all saved command lines and dependencies for the $(targets) we
 # may be building above, using $(if_changed{,_dep}). As an
 # optimization, we don't need to read them if the target does not