cpu/mpc824x/Makefile: fix warning with parallel builds

Parallel builds would occasionally issue this build warning:

    ln: creating symbolic link `cpu/mpc824x/bedbug_603e.c': File exists

Use "ln -sf" as quick work around for the issue.

Signed-off-by: Wolfgang Denk <wd@denx.de>

cpu/mpc824x/Makefile

@@ -44,7 +44,7 @@ $(LIB):	$(OBJS)
 	$(AR) $(ARFLAGS) $@ $(OBJS)
 
 $(obj)bedbug_603e.c:
-	ln -s $(src)../mpc8260/bedbug_603e.c $(obj)bedbug_603e.c
+	ln -sf $(src)../mpc8260/bedbug_603e.c $(obj)bedbug_603e.c
 
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