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/droiddoc.mk b/core/droiddoc.mk
index aae2aba..893461a 100644
--- a/core/droiddoc.mk
+++ b/core/droiddoc.mk
@@ -23,6 +23,7 @@
 LOCAL_IS_HOST_MODULE := $(call true-or-empty,$(LOCAL_IS_HOST_MODULE))
 ifeq ($(LOCAL_IS_HOST_MODULE),true)
 my_prefix := HOST_
+LOCAL_HOST_PREFIX :=
 else
 my_prefix := TARGET_
 endif