support/download/file: fix file:// protocol handling

support/download/file: fix file:// protocol handling

Since the rework of the download infrastructure, the "file" download
helper gets passed an URL that starts with file://, but forgets to
strip it before passing it to "cp", causing a failure as the "cp"
program isn't prepared for file paths starting with file://. This is
fixed by stripping the file:// at the beginning of the URL.

In addition, the path passed to cp lacked a slash between the
directory path and the filename part of the url. This is fixed by
adding a slash at the appropriate places.

Fixes the following build failure when the "file" download method is
used:

cp: cannot stat 'file:///home/angelo/DEV/TOOLCHAINSarmv7-eabihf--glibc--bleeding-edge-2017.11-1.tar.bz2': No such file or directory

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>