SDK: source.properties templates.

This adds support for "templates" source.properties files that can
inherit the platform version/codename/api level from the build.

The purpose is for a given build to always generate SDK files that
match the platform generated in the same branch.

Change-Id: Id623bbef45a04d953da8eade7e8c34c77709f9c3
diff --git a/build/sdk-android-armeabi-v7a.atree b/build/sdk-android-armeabi-v7a.atree
index 8867bf7..e586945 100644
--- a/build/sdk-android-armeabi-v7a.atree
+++ b/build/sdk-android-armeabi-v7a.atree
@@ -18,4 +18,4 @@
 prebuilt/android-${TARGET_ARCH}/kernel/kernel-qemu-armv7 system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/kernel-qemu
 
 # version files for the SDK updater, from development.git
-development/sdk/images_armeabi-v7a_source.properties   system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/source.properties
+${HOST_OUT}/development/sdk/images_armeabi-v7a_source.properties system-images/${PLATFORM_NAME}/${TARGET_CPU_ABI}/source.properties