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/board/freescale/mx51evk/config.mk b/board/freescale/mx51evk/config.mk
index c8279ec..af70ec2 100644
--- a/board/freescale/mx51evk/config.mk
+++ b/board/freescale/mx51evk/config.mk
@@ -20,6 +20,6 @@
 # MA 02111-1307 USA
 #
 
-LDSCRIPT = cpu/$(CPU)/$(SOC)/u-boot.lds
+LDSCRIPT = $(CPUDIR)/$(SOC)/u-boot.lds
 TEXT_BASE = 0x97800000
 IMX_CONFIG = $(SRCTREE)/board/$(BOARDDIR)/imximage.cfg