discover/paths: Parse Busybox progress information

Several busybox utilities (tftp and wget in particular) use a common
format for progress bar output. Add a stdout callback that recognises
this format and passes progress information to
device_handler_status_download().
If Petitboot has been explicitly built with busybox support set
busybox_progress_cb() as the default stdout callback for
load_url_async().

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