Merge changes from topic 'nyc-mr1-ue_sideload_recovery' into nyc-mr1-dev
* changes:
Setup a temporary directory for update_engine_sideload.
DO NOT MERGE: Make update_engine compile in the branch.
New setting to mark postinstall as optional.
Report the progress of the update when sideloading.
Compile update_engine_sideload as a static recovery program.
Remove libcurl support from update_engine_sideload.
Build update_engine_sideload.
Implement a memory-based Prefs class.
Remove unused libbrillo-http dependency.