support/download/scp: fix download with scheme prefix 'scp://'

The scp download helper is broken when the server URL starts with 'scp://'.
Such prefix is used in two situations:
1. to let FOO_SITE point to an scp location without explicitly having to set
   'FOO_SITE_METHOD = scp'

2. when BR2_PRIMARY_SITE or BR2_BACKUP_SITE points to an scp location. In
   this case, there is no equivalent of 'SITE_METHOD'.

Strip out the scheme prefix, similarly to how the 'file' download helper
does it. That helper has the same cases as above.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/download/scp

@@ -34,4 +34,7 @@ _scp() {
     eval ${SCP} "${@}"
 }
 
+# Remove any scheme prefix
+uri="${uri##scp://}"
+
 _scp ${verbose} "${@}" "'${uri}/${filename}'" "'${output}'"