GENTARGETS: add support for scp://

This patch adds support for scp:// both for use in the package Makefiles, as for
the BR2_PRIMARY_SITE variable.

This patch was based on the work of Richard Guy Briggs
(see https://bugs.busybox.net/show_bug.cgi?id=3343

).

[Peter: small whitespace fixes]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>