Support building Windows SDK under Linux.

- envsetup.mk & config.mk: we define a new BUILD_OS and a minimal set
of things like BUILD_OUT to be able to use some local tools when
doing cross-compilation. This allows us to use the Linux version of
ACP when cross-compiling the tools to Windows.

- Makfile: include windows_sdk.mk when needed to build a Windows SDK.

- main.mk: support a win_sdk target (e.g. PRODUCT-sdk-win_sdk)

(Merge master Change I9d08d0df)
diff --git a/core/main.mk b/core/main.mk
index 014c050..f830f03 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -179,6 +179,9 @@
 ifneq ($(filter sdk,$(MAKECMDGOALS)),)
 is_sdk_build := true
 endif
+ifneq ($(filter win_sdk,$(MAKECMDGOALS)),)
+is_sdk_build := true
+endif
 ifneq ($(filter sdk_addon,$(MAKECMDGOALS)),)
 is_sdk_build := true
 endif
@@ -254,6 +257,7 @@
 ifneq ($(words $(filter-out $(INTERNAL_MODIFIER_TARGETS),$(MAKECMDGOALS))),1)
 $(error The 'sdk' target may not be specified with any other targets)
 endif
+
 # TODO: this should be eng I think.  Since the sdk is built from the eng
 # variant.
 tags_to_install := user debug eng