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/nand_spl/board/freescale/mpc8536ds/Makefile b/nand_spl/board/freescale/mpc8536ds/Makefile
index 7ed9d61..be0caaa 100644
--- a/nand_spl/board/freescale/mpc8536ds/Makefile
+++ b/nand_spl/board/freescale/mpc8536ds/Makefile
@@ -29,7 +29,7 @@
 
 include $(TOPDIR)/config.mk
 
-LDSCRIPT= $(TOPDIR)/cpu/$(CPU)/u-boot-nand_spl.lds
+LDSCRIPT= $(TOPDIR)/$(CPUDIR)/u-boot-nand_spl.lds
 LDFLAGS	= -Bstatic -T $(LDSCRIPT) -Ttext $(TEXT_BASE) $(PLATFORM_LDFLAGS)
 AFLAGS	+= -DCONFIG_NAND_SPL
 CFLAGS	+= -DCONFIG_NAND_SPL
@@ -93,7 +93,7 @@
 
 $(obj)resetvec.S:
 	@rm -f $(obj)resetvec.S
-	ln -s $(SRCTREE)/cpu/$(CPU)/resetvec.S $(obj)resetvec.S
+	ln -s $(SRCTREE)/$(CPUDIR)/resetvec.S $(obj)resetvec.S
 
 $(obj)fixed_ivor.S:
 	@rm -f $(obj)fixed_ivor.S