Build SDK repository

When the build is invoked with the fake target "sdk_repo" and
a main target of sdk, win_sdk or sdk_addon, we now create
packages in DIST_DIR that can directly be used to populate the
SDK Repository.
This is quite close to how we actually distribute the SDK.

Change-Id: Ie7e0f789bafd73d5ee2461349155a6e562e2edd9