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/lib_arm/config.mk b/lib_arm/config.mk
index 3c078df..8dd4dcc 100644
--- a/lib_arm/config.mk
+++ b/lib_arm/config.mk
@@ -63,4 +63,4 @@
 PLATFORM_LIBS += $(OBJTREE)/lib_arm/eabi_compat.o
 endif
 endif
-LDSCRIPT := $(SRCTREE)/cpu/$(CPU)/u-boot.lds
+LDSCRIPT := $(SRCTREE)/$(CPUDIR)/u-boot.lds