s390: add custom target and make path extension optional for .bss check

"chkbss-target" could be now used to override default target .bss check
is bound to. Objects to be checked are only path extended when needed.
Reviewed-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

@@ -8,7 +8,12 @@ define cmd_chkbss
 	touch $@;
 endef
 
-$(obj)/built-in.a: $(patsubst %, $(obj)/%.chkbss, $(chkbss))
+chkbss-target ?= $(obj)/built-in.a
+ifneq (,$(findstring /,$(chkbss)))
+$(chkbss-target): $(patsubst %, %.chkbss, $(chkbss))
+else
+$(chkbss-target): $(patsubst %, $(obj)/%.chkbss, $(chkbss))
+endif
 
 %.o.chkbss: %.o
 	$(call cmd,chkbss)