Add HOST_CROSS_OS

Instead of using recursive make to change the HOST_OS when building the
windows SDK under linux, add the concept of cross-building to another
host os.

Bug: 23566667
Change-Id: I6dc525b601b6251d458d197c30bf4660d7485502
diff --git a/core/dumpvar.mk b/core/dumpvar.mk
index 510bc7d..c7fb8db 100644
--- a/core/dumpvar.mk
+++ b/core/dumpvar.mk
@@ -80,6 +80,7 @@
 $(info   HOST_ARCH=$(HOST_ARCH))
 $(info   HOST_OS=$(HOST_OS))
 $(info   HOST_OS_EXTRA=$(HOST_OS_EXTRA))
+$(info   HOST_CROSS_OS=$(HOST_CROSS_OS))
 $(info   HOST_BUILD_TYPE=$(HOST_BUILD_TYPE))
 $(info   BUILD_ID=$(BUILD_ID))
 $(info   OUT_DIR=$(OUT_DIR))