s390/build: add *.o.chkbss files to targets list

Adding *.o.chkbss files to targets list makes sure that the kbuild is
aware of them and removes them during make clean.

Also remove *.o.chkbss file before an actual check, to avoid having
stale *.o.chkbss file left even if the check is failed.
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/scripts/Makefile.chkbss

@@ -2,6 +2,7 @@
 
 quiet_cmd_chkbss = CHKBSS  $<
 define cmd_chkbss
+	rm -f $@; \
 	if ! $(OBJDUMP) -j .bss -w -h $< | awk 'END { if ($$3) exit 1 }'; then \
 		echo "error: $< .bss section is not empty" >&2; exit 1; \
 	fi; \
@@ -10,10 +11,13 @@ endef
 
 chkbss-target ?= $(obj)/built-in.a
 ifneq (,$(findstring /,$(chkbss)))
-$(chkbss-target): $(patsubst %, %.chkbss, $(chkbss))
+chkbss-files := $(patsubst %, %.chkbss, $(chkbss))
 else
-$(chkbss-target): $(patsubst %, $(obj)/%.chkbss, $(chkbss))
+chkbss-files := $(patsubst %, $(obj)/%.chkbss, $(chkbss))
 endif
 
+$(chkbss-target): $(chkbss-files)
+targets += $(notdir $(chkbss-files))
+
 %.o.chkbss: %.o
 	$(call cmd,chkbss)