Starting with Tools R12, the SDK Manager offers a slightly better way to update the SDK from command-line.
Previously the "android update sdk --no-ui" command already allowed one to update from the command-line, but it had the annoying tendency of installing every single platform or add-on. There are now 30 or more packages available on the SDK repository, including 12 platforms; that's a lot to download when most people only need the most recent platform. Starting with R12, you can use combine new things:
There's still room for improvement, notably in a future release we'll work on the first install case and making the update smarter at figuring out what to get.