Create CPUDIR variable

The CPUDIR variable points to the location of a target's CPU directory.
Currently, it is set to cpu/$CPU.  However, using $CPUDIR will allow for
more flexibility in the future.  It lays the groundwork for reorganizing
U-Boot's directory structure to support a layout such as:

  arch/$ARCH/cpu/$CPU/* (architecture with multiple CPU types)
  arch/$ARCH/cpu/*      (architecture with one CPU type)

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
diff --git a/onenand_ipl/board/apollon/Makefile b/onenand_ipl/board/apollon/Makefile
index 193efff..6f1df01 100644
--- a/onenand_ipl/board/apollon/Makefile
+++ b/onenand_ipl/board/apollon/Makefile
@@ -46,7 +46,7 @@
 # from cpu directory
 $(obj)start.S:
 	@rm -f $@
-	ln -s $(SRCTREE)/cpu/$(CPU)/start.S $@
+	ln -s $(SRCTREE)/$(CPUDIR)/start.S $@
 
 # from onenand_ipl directory
 $(obj)onenand_ipl.h: