discover/device-handler: Add aggregated download progress updates

Several processes run by Petitboot output progress information while
running. Add device_handler_status_download() which process callers can
call to register and update progress information (percentage and current
size).
A list of 'progress_info' structs holds this progress information, and
on each call to device_handler_status_download() the information is
combined and displayed as a single status update for readability.
On completion device_handler_status_download_remove() is called to
remove old progress information from the list.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>