kbuild: Allow .dtbo overlays to be built piecemeal

raspberrypi inclusion
category: feature
bugzilla: 50432

--------------------------------

Before 4.20, it was possible to build an arbitrary overlay by copying
it to arm/boot/dts/overlays/mytest-overlay.dts and running:

    make ARCH=arm overlays/mytest.dtbo

In 4.20 the .dtb build rules were centralised, requiring the dowstream
.dtbo build rules to be changed. They were, enough to support "make ...
dtbs", but not sufficiently to allow this ad-hoc, one-off building of
individual files.

Add the missing makefile rule to support this way of building.

See: https://github.com/raspberrypi/linux/issues/3250Signed-off-by: NPhil Elwell <phil@raspberrypi.org>
Signed-off-by: NFang Yafen <yafen@iscas.ac.cn>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

Makefile

@@ -1333,6 +1333,9 @@ ifneq ($(dtstree),)
 %.dtb: include/config/kernel.release scripts_dtc
 	$(Q)$(MAKE) $(build)=$(dtstree) $(dtstree)/$@
 
+%.dtbo: prepare3 scripts_dtc
+	$(Q)$(MAKE) $(build)=$(dtstree) $(dtstree)/$@
+
 PHONY += dtbs dtbs_install dtbs_check
 dtbs: include/config/kernel.release scripts_dtc
 	$(Q)$(MAKE) $(build)=$(dtstree)