updates: Add script for enabling the official update signing key.

This script should be called before running build_image when generating
official production images. Images built with official key will not
accept updates signed with the default development signing key.
build_image: check that set_official wasn't forgotten in official builds
updates: extract usr partition when building images

The current generate_update function is now less useful, the important
part that we need is just the partition image now. Also by defaulting to
extracting the partition the old cors_generate_update which is still in
use by devserver can be removed entirely, devserver will just expect the
extracted partition image instead.
updates: use HTTPS for fetching update payloads
Merge pull request #283 from marineam/version2

updates: Add script for enabling the official update signing key.