devserver: in case archive_dir != static_dir, serve from archive_dir
devserver: in case archive_dir != static_dir, serve from archive_dir

This was ommited from 608f46c3 which broke some workflows.

Change-Id: Ic85ca6f52eda0c95b254a010fe6cd507da893fe3

BUG=5246
TEST=start devserver and see serving from $archive_dir instead

Review URL: http://codereview.chromium.org/6591113

mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,coreos/dev-util