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/voiceblue/config.mk b/board/voiceblue/config.mk
index b77c91c..d55daa4 100644
--- a/board/voiceblue/config.mk
+++ b/board/voiceblue/config.mk
@@ -3,7 +3,7 @@
 # entry 1000'8000 (mem base + reserved)
 #
 
-sinclude $(TOPDIR)/board/$(BOARDDIR)/config.tmp
+sinclude $(OBJTREE)/board/$(BOARDDIR)/config.tmp
 
 ifeq ($(VOICEBLUE_SMALL_FLASH),y)
 # We load ourself to internal SRAM at 2001'2000