kbuild: drop $(objtree)/ prefix support for clean-files

I think this hack is a bad idea. arch/powerpc/boot/Makefile is the
only and last user. Let's stop doing this.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)

arch/powerpc/boot/Makefile

@@ -453,8 +453,8 @@ clean-files += $(image-) $(initrd-) cuImage.* dtbImage.* treeImage.* \
 clean-kernel-base := vmlinux.strip vmlinux.bin
 clean-kernel := $(addsuffix .gz,$(clean-kernel-base))
 clean-kernel += $(addsuffix .xz,$(clean-kernel-base))
-# If not absolute clean-files are relative to $(obj).
-clean-files += $(addprefix $(objtree)/, $(clean-kernel))
+# clean-files are relative to $(obj).
+clean-files += $(addprefix ../../../, $(clean-kernel))
 
 WRAPPER_OBJDIR := /usr/lib/kernel-wrapper
 WRAPPER_DTSDIR := /usr/lib/kernel-wrapper/dts

scripts/Makefile.clean

@@ -36,13 +36,7 @@ __clean-files	:= \
 
 __clean-files   := $(filter-out $(no-clean-files), $(__clean-files))
 
-# clean-files is given relative to the current directory, unless it
-# starts with $(objtree)/ (which means "./", so do not add "./" unless
-# you want to delete a file from the toplevel object directory).
-
-__clean-files   := $(wildcard                                               \
-		   $(addprefix $(obj)/, $(filter-out $(objtree)/%, $(__clean-files))) \
-		   $(filter $(objtree)/%, $(__clean-files)))
+__clean-files   := $(wildcard $(addprefix $(obj)/, $(__clean-files)))
 
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