Add support for a saving build objects in a separate directory.
Modifications are based on the linux kernel approach and
support two use cases:

  1) Add O= to the make command line
  'make O=/tmp/build all'

  2) Set environement variable BUILD_DIR to point to the desired location
  'export BUILD_DIR=/tmp/build'
  'make'

The second approach can also be used with a MAKEALL script
'export BUILD_DIR=/tmp/build'
'./MAKEALL'

Command line 'O=' setting overrides BUILD_DIR environent variable.

When none of the above methods is used the local build is performed and
the object files are placed in the source directory.
diff --git a/board/mpc8260ads/config.mk b/board/mpc8260ads/config.mk
index eb6f7c9..e99e181 100644
--- a/board/mpc8260ads/config.mk
+++ b/board/mpc8260ads/config.mk
@@ -29,7 +29,7 @@
 # MPC8260ADS, MPC8266ADS, and PQ2FADS-ZU/VR boards
 #
 
-sinclude $(TOPDIR)/board/$(BOARDDIR)/config.tmp
+sinclude $(OBJTREE)/board/$(BOARDDIR)/config.tmp
 
 ifndef TEXT_BASE
 ## Standard: boot high