Allow to set ANDROID_BUILDSPEC as path of buildspec.mk

So that clients can set a custom buildspec.mk without adding
buildspec.mk at the top dir.

Change-Id: Ic15efa6cfbf00f04db35447e70bb5bdd63a5058c
diff --git a/core/config.mk b/core/config.mk
index 41e60ea..d486ed3 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -117,7 +117,10 @@
 # If this file doesn't exist, the environemnt variables will
 # be used, and if that doesn't work, then the default is an
 # arm build
--include $(TOPDIR)buildspec.mk
+ifndef ANDROID_BUILDSPEC
+ANDROID_BUILDSPEC := $(TOPDIR)buildspec.mk
+endif
+-include $(ANDROID_BUILDSPEC)
 
 # ---------------------------------------------------------------
 # Define most of the global variables.  These are the ones that