support/download: fix scp downloads

support/download: fix scp downloads

scp download is broken, because scp is called without filename argument and
only the server is specified. The call is:
    scp <server> <outputfile>

but should be:
    scp <server>/<filename> <outputfile>

Instead of assuming '-u' lists a full URL including filename (which it is
not), align with the wget helper where -u is the server URL and -f gives the
filename.

With this commit, an scp download can work if FOO_SITE_METHOD is explicitly
set to 'scp' and the server does not have a scheme prefix 'scp://'.
The next commit will handle the case where a scheme prefix is present.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: s/URL/URI/, as noticed by Yann.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>