sh: Fix up rsk7203 target for out of tree build

Fix up rsk7203 target to build successfully using out-of-tree build.

Signed-off-by: Kieran Bingham <kbingham@mpc-data.co.uk>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
diff --git a/cpu/sh2/Makefile b/cpu/sh2/Makefile
index e33ba0f..346d328 100644
--- a/cpu/sh2/Makefile
+++ b/cpu/sh2/Makefile
@@ -28,18 +28,23 @@
 
 LIB	= $(obj)lib$(CPU).a
 
-START	= start.o
-OBJS	= cpu.o interrupts.o watchdog.o
+SOBJS	= start.o
+COBJS	= cpu.o interrupts.o watchdog.o
 
-all:	.depend $(START) $(LIB)
+SRCS    := $(SOBJS:.o=.S) $(COBJS:.o=.c)
+OBJS    := $(addprefix $(obj),$(COBJS))
+SOBJS   := $(addprefix $(obj),$(SOBJS))
 
-$(LIB):	$(OBJS)
-	$(AR) $(ARFLAGS) $@ $(OBJS)
+$(LIB):	$(OBJS) $(SOBJS)
+	$(AR) $(ARFLAGS) $@ $(OBJS) $(SOBJS)
 
-#########################################################################
+clean:
+	rm -f $(SOBJS) $(OBJS)
 
-.depend:	Makefile $(START:.o=.S) $(OBJS:.o=.c)
-		$(CC) -M $(CFLAGS) $(START:.o=.S) $(OBJS:.o=.c) > $@
+distclean:	clean
+	rm -f $(LIB) core *.bak $(obj).depend
+
+#########################################################################
 
 # defines $(obj).depend target
 include $(SRCTREE)/rules.mk