Blackfin: only build post code when CONFIG_POST

Save some time by using CONFIG_POST in the Makefile rather than C files.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/lib_blackfin/Makefile b/lib_blackfin/Makefile
index fee0fda..93f7d4f 100644
--- a/lib_blackfin/Makefile
+++ b/lib_blackfin/Makefile
@@ -40,9 +40,8 @@
 COBJS-y	+= boot.o
 COBJS-y	+= cache.o
 COBJS-y	+= muldi3.o
-COBJS-y	+= post.o
+COBJS-$(CONFIG_POST) += post.o tests.o
 COBJS-y	+= string.o
-COBJS-y	+= tests.o
 
 SRCS	:= $(SOBJS-y:.o=.S) $(COBJS-y:.o=.c)
 OBJS	:= $(addprefix $(obj),$(SOBJS-y) $(COBJS-y))