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/config.mk b/config.mk
index cb1c4af..c82cb63 100644
--- a/config.mk
+++ b/config.mk
@@ -110,14 +110,15 @@
 # Load generated board configuration
 sinclude $(OBJTREE)/include/autoconf.mk
 
-ifdef	ARCH
+# Some architecture config.mk files need to know what CPUDIR is set to,
+# so calculate CPUDIR before including ARCH/SOC/CPU config.mk files.
+CPUDIR=cpu/$(CPU)
+
 sinclude $(TOPDIR)/lib_$(ARCH)/config.mk	# include architecture dependend rules
-endif
-ifdef	CPU
-sinclude $(TOPDIR)/cpu/$(CPU)/config.mk		# include  CPU	specific rules
-endif
+sinclude $(TOPDIR)/$(CPUDIR)/config.mk		# include  CPU	specific rules
+
 ifdef	SOC
-sinclude $(TOPDIR)/cpu/$(CPU)/$(SOC)/config.mk	# include  SoC	specific rules
+sinclude $(TOPDIR)/$(CPUDIR)/$(SOC)/config.mk	# include  SoC	specific rules
 endif
 ifdef	VENDOR
 BOARDDIR = $(VENDOR)/$(BOARD)