kbuild: Allow .dtbo overlays to be built piecemeal

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: #3250

Signed-off-by: Phil Elwell <phil@raspberrypi.org>

Makefile

@@ -1258,6 +1258,9 @@ ifneq ($(dtstree),)
 %.dtb: prepare3 scripts_dtc
 	$(Q)$(MAKE) $(build)=$(dtstree) $(dtstree)/$@
 
+%.dtbo: prepare3 scripts_dtc
+	$(Q)$(MAKE) $(build)=$(dtstree) $(dtstree)/$@
+
 PHONY += dtbs dtbs_install dt_binding_check
 dtbs dtbs_check: prepare3 scripts_dtc
 	$(Q)$(MAKE) $(build)=$(dtstree)