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/config.mk b/core/config.mk
index 3eddc13..a4d3154 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -195,7 +195,6 @@
 YACC:= bison -d
 DOXYGEN:= doxygen
 AAPT := $(HOST_OUT_EXECUTABLES)/aapt$(HOST_EXECUTABLE_SUFFIX)
-ACP := $(HOST_OUT_EXECUTABLES)/acp$(HOST_EXECUTABLE_SUFFIX)
 AIDL := $(HOST_OUT_EXECUTABLES)/aidl$(HOST_EXECUTABLE_SUFFIX)
 ICUDATA := $(HOST_OUT_EXECUTABLES)/icudata$(HOST_EXECUTABLE_SUFFIX)
 SIGNAPK_JAR := $(HOST_OUT_JAVA_LIBRARIES)/signapk$(COMMON_JAVA_PACKAGE_SUFFIX)
@@ -214,6 +213,9 @@
 PROGUARD := external/proguard/bin/proguard.sh
 JAVATAGS := build/tools/java-event-log-tags.py
 
+# ACP is always for the build OS, not for the host OS
+ACP := $(BUILD_OUT_EXECUTABLES)/acp$(BUILD_EXECUTABLE_SUFFIX)
+
 # dx is java behind a shell script; no .exe necessary.
 DX := $(HOST_OUT_EXECUTABLES)/dx
 KCM := $(HOST_OUT_EXECUTABLES)/kcm$(HOST_EXECUTABLE_SUFFIX)