kbuild: use $(obj)/ instead of $(src)/ for common pattern rules

Kbuild conventionally uses $(obj)/ for generated files, and $(src)/ for
checked-in source files. It is merely a convention without any functional
difference. In fact, $(obj) and $(src) are exactly the same, as defined
in scripts/Makefile.build:

  src := $(obj)

Before changing the semantics of $(src) in the next commit, this commit
replaces $(obj)/ with $(src)/ in pattern rules where the prerequisite
might be a generated file.

C, assembly, Rust, and DTS files are sometimes generated by tools, so
they could be either generated files or real sources. The $(obj)/ prefix
works for both cases with the help of VPATH.

As mentioned above, $(obj) and $(src) are the same at this point, hence
this commit has no functional change.

I did not modify scripts/Makefile.userprogs because there is no use
case where userspace C files are generated.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

scripts/Makefile.lib

@@ -423,7 +423,7 @@ quiet_cmd_dtb = $(quiet_cmd_dtc)
       cmd_dtb = $(cmd_dtc)
 endif
 
-$(obj)/%.dtb: $(src)/%.dts $(DTC) $(DT_TMP_SCHEMA) FORCE
+$(obj)/%.dtb: $(obj)/%.dts $(DTC) $(DT_TMP_SCHEMA) FORCE
 	$(call if_changed_dep,dtb)
 
 $(obj)/%.dtbo: $(src)/%.dtso $(DTC) FORCE