ARC: [kbuild] Avoid DTB rebuilds if DTS are untouched

Currently, for every ARC kernel build I see the following:

--------------->8-----------------
DTB arch/arc/boot/dts/angel4.dtb.S
AS arch/arc/boot/dts/angel4.dtb.o
LD arch/arc/boot/dts/built-in.o
rm arch/arc/boot/dts/angel4.dtb.S <-- forces rebuild next iter
CHK kernel/config_data.h
--------------->8-----------------

This is because *.dts.S is intermediate file in dtb generation and is by
default deleted by make which needs a ".SECONDARY" hint to NOT do so.

This could have ideally been done in scripts/Makefile.lib - for benefit
of all, however .SECONDARY doesn't seem to work with wildcards.

Thanks to Stephen for suggesting .SECONDARY (vs .PRECIOUS) and making
that work using a non wildcard version in arch makefile.

Thanks to James Hogan for pointing out that *.dtb.S now needs to be
added to clean-files

Signed-off-by: Stephen Warren <[e-mail]>
Signed-off-by: Vineet Gupta <[e-mail]>

arch/arc/boot/dts/Makefile

···
 obj-y   += $(builtindtb-y).dtb.o
 targets += $(builtindtb-y).dtb
 
+.SECONDARY: $(obj)/$(builtindtb-y).dtb.S
+
 dtbs:  $(addprefix  $(obj)/, $(builtindtb-y).dtb)
 
-clean-files := *.dtb
+clean-files := *.dtb  *.dtb.S