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/doc/README.ARM-SoC b/doc/README.ARM-SoC
index 0f2677f..15f065d 100644
--- a/doc/README.ARM-SoC
+++ b/doc/README.ARM-SoC
@@ -7,14 +7,14 @@
 directory. That means that code that is cpu (i.e. core) specific
 resides in
 
-	cpu/$(CPU)/
+	$(CPUDIR)/
 
 and code that is specific to some SoC (i.e. vendor specific
 peripherals around the core) is moved into
 
-	cpu/$(CPU)/$(SOC)/
+	$(CPUDIR)/$(SOC)/
 
-Thus a library/archive "cpu/$(CPU)/$(SOC)/lib$(SOC).a" will be build
+Thus a library/archive "$(CPUDIR)/$(SOC)/lib$(SOC).a" will be build
 and linked. Examples will be
 
 	cpu/arm920t/imx/