as $(obj) is empty when in tree build

%.s:	%.S
	$(CPP) $(AFLAGS) -o $@ $<

and

$(obj)%.s:	%.S
	$(CPP) $(AFLAGS) -o $@ $<

are the same

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>