kbuild: Fix removal of the debian/ directory

scripts/Makefile.clean treats absolute path specially, but
$(objtree)/debian is no longer an absolute path since 7e1c0477 (kbuild:
Use relative path for $(objtree). Work around this by checking if the
path starts with $(objtree)/.

Reported-and-tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Fixes: 7e1c0477 (kbuild: Use relative path for $(objtree)
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/Makefile.clean

@@ -42,19 +42,19 @@ __clean-files	:= $(extra-y) $(extra-m) $(extra-)       \
 
 __clean-files   := $(filter-out $(no-clean-files), $(__clean-files))
 
-# as clean-files is given relative to the current directory, this adds
-# a $(obj) prefix, except for absolute paths
+# 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 /%, $(__clean-files))) \
-		   $(filter /%, $(__clean-files)))
+		   $(addprefix $(obj)/, $(filter-out $(objtree)/%, $(__clean-files))) \
+		   $(filter $(objtree)/%, $(__clean-files)))
 
-# as clean-dirs is given relative to the current directory, this adds
-# a $(obj) prefix, except for absolute paths
+# same as clean-files
 
 __clean-dirs    := $(wildcard                                               \
-                   $(addprefix $(obj)/, $(filter-out /%, $(clean-dirs)))    \
-		   $(filter /%, $(clean-dirs)))
+		   $(addprefix $(obj)/, $(filter-out $(objtree)/%, $(clean-dirs)))    \
+		   $(filter $(objtree)/%, $(clean-dirs)))
 
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