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)