kbuild: Really don't clean bounds.h and asm-offsets.h

Commit 7d3cc8b tried to keep bounds.h and asm-offsets.h during make
clean by filtering these out of $(clean-files), but they are listed in
$(targets) and $(always) and thus removed automatically. Introduce a new
$(no-clean-files) variable to really skip such files in Makefile.clean.

Signed-off-by: Michal Marek <[e-mail]>

Documentation/kbuild/makefiles.txt

···
 Kbuild will assume the directories to be in the same relative path as the
 Makefile if no absolute path is specified (path does not start with '/').
 
+To exclude certain files from make clean, use the $(no-clean-files) variable.
+This is only a special case used in the top level Kbuild file:
+
+	Example:
+		#Kbuild
+		no-clean-files := $(bounds-file) $(offsets-file)
+
 Usually kbuild descends down in subdirectories due to "obj-* := dir/",
 but in the architecture makefiles where the kbuild infrastructure
 is not sufficient this sometimes needs to be explicit.

Kbuild

···
 missing-syscalls: scripts/checksyscalls.sh FORCE
 	$(call cmd,syscalls)
 
-# Delete all targets during make clean
-clean-files := $(addprefix $(objtree)/,$(filter-out $(bounds-file) $(offsets-file),$(targets)))
+# Keep these two files during make clean
+no-clean-files := $(bounds-file) $(offsets-file)

scripts/Makefile.clean

···
 		   $(host-progs)                         \
 		   $(hostprogs-y) $(hostprogs-m) $(hostprogs-)
 
+__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