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_sh/config.mk b/lib_sh/config.mk
index fa5369f..797bf4c 100644
--- a/lib_sh/config.mk
+++ b/lib_sh/config.mk
@@ -31,4 +31,4 @@
 PLATFORM_CPPFLAGS += -DCONFIG_SH -D__SH__
 PLATFORM_LDFLAGS += -e $(TEXT_BASE) --defsym reloc_dst=$(TEXT_BASE)
 
-LDSCRIPT := $(SRCTREE)/cpu/$(CPU)/u-boot.lds
+LDSCRIPT := $(SRCTREE)/$(CPUDIR)/u-boot.lds