kbuild: Allow DTB overlays to built into .dtbo.S files

DTB files can be built into the kernel by converting them to assembly
files then assembling them into object files. We extend this here
for DTB overlays with the .dtso extensions.

We change the start and end delimiting tag prefix to make it clear that
this data came from overlay files.

[Based on patch by Frank Rowand <frank.rowand@sony.com>]

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Tested-by: Frank Rowand <frowand.list@gmail.com>
Link: https://lore.kernel.org/r/20221024173434.32518-3-afd@ti.com
Signed-off-by: Rob Herring <robh@kernel.org>

scripts/Makefile.lib

@@ -358,7 +358,7 @@ DTC_FLAGS += $(DTC_FLAGS_$(basetarget))
 DTC_FLAGS += $(if $(filter $(patsubst $(obj)/%,%,$@), $(base-dtb-y)), -@)
 
 # Generate an assembly file to wrap the output of the device tree compiler
-quiet_cmd_dt_S_dtb= DTB     $@
+quiet_cmd_dt_S_dtb= DTBS    $@
 cmd_dt_S_dtb=						\
 {							\
 	echo '\#include <asm-generic/vmlinux.lds.h>'; 	\
@@ -375,6 +375,24 @@ cmd_dt_S_dtb=						\
 $(obj)/%.dtb.S: $(obj)/%.dtb FORCE
 	$(call if_changed,dt_S_dtb)
 
+# Generate an assembly file to wrap the output of the device tree compiler
+quiet_cmd_dt_S_dtbo= DTBOS   $@
+cmd_dt_S_dtbo=						\
+{							\
+	echo '\#include <asm-generic/vmlinux.lds.h>';	\
+	echo '.section .dtb.init.rodata,"a"';		\
+	echo '.balign STRUCT_ALIGNMENT';		\
+	echo '.global __dtbo_$(subst -,_,$(*F))_begin';	\
+	echo '__dtbo_$(subst -,_,$(*F))_begin:';	\
+	echo '.incbin "$<" ';				\
+	echo '__dtbo_$(subst -,_,$(*F))_end:';		\
+	echo '.global __dtbo_$(subst -,_,$(*F))_end';	\
+	echo '.balign STRUCT_ALIGNMENT';		\
+} > $@
+
+$(obj)/%.dtbo.S: $(obj)/%.dtbo FORCE
+	$(call if_changed,dt_S_dtbo)
+
 quiet_cmd_dtc = DTC     $@
 cmd_dtc = $(HOSTCC) -E $(dtc_cpp_flags) -x assembler-with-cpp -o $(dtc-tmp) $< ; \
 	$(DTC) -o $@ -b 0 \