scripts: download.pl: retry download using filename

scripts: download.pl: retry download using filename

With this commit, the download script will try downloading source files
using the filename instead of the url-filename in case the previous
download attempt using the url-filename failed.

This is required, as the OpenWrt sources mirrors serve files using the
filename files might be renamed to after downloading. If the original
mirror for a file where url-filename and filename do not match goes
down, the download failed prior to this patch.

Further improvement can be done by performing this only for the
OpenWrt sources mirrors.

Signed-off-by: David Bauer <mail@david-bauer.net>