[PATCH master] kbuild: don't expect a Kbuild file in dts/src

[Ahmad Fatoum <a.fatoum@barebox.org>]

dts/ is synced from upstream as is, so the v7.3 sync dropped the empty
dts/src/Kbuild added by commit 77addf6205d4 ("kbuild: allow building
the device trees in dts/ by name") and building a device tree by name
fails since:

  make[2]: *** No rule to make target 'dts/src/Makefile'.  Stop.

Let's have Makefile.build include /dev/null instead. Overlays never
built by name at all, as Makefile.build pulls in Makefile.dtbs without
a dtb-y only for .dtb goals, so set need-dtbslist for them too.

Fixes: 77addf6205d4 ("kbuild: allow building the device trees in dts/ by name")
Assisted-by: Claude:opus-5.5
Signed-off-by: Ahmad Fatoum <a.fatoum@barebox.org>
---
 Makefile | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index c3de0956eb7c..8f3f7bc17c25 100644
--- a/Makefile
+++ b/Makefile
@@ -1444,11 +1444,13 @@ ifneq ($(dtstree),)
 
 # Build an upstream device tree by name, e.g.
 #   make dts/src/arm/st/stm32mp157c-dk2.dtb
+# dts/ is synced from upstream as is, so there's no Kbuild file to include
 dts/src/%.dtb: dtbs_prepare
-	$(Q)$(MAKE) $(build)=dts/src $@
+	$(Q)$(MAKE) $(build)=dts/src kbuild-file=/dev/null $@
 
+# without a dtb-y, Makefile.build includes Makefile.dtbs only for .dtb goals
 dts/src/%.dtbo: dtbs_prepare
-	$(Q)$(MAKE) $(build)=dts/src $@
+	$(Q)$(MAKE) $(build)=dts/src kbuild-file=/dev/null need-dtbslist=y $@
 
 PHONY += dtbs dtbs_prepare dtbs_install
 dtbs: dtbs_prepare
-- 
2.47.3