HBSD: use fetch_update for 'download only'
We now use the fetch_update for 'download only' function. This function has the advantage to control the checksum of the download file. Please note that there is no more resumption of the interrupted download. Reported-by: Shawn Webb <firstname.lastname@example.org> Signed-off-by: Loic <email@example.com> (cherry picked from commit b61c6a82)