pkg-download: fix svn download for deleted path

If a SVN path has been deleted in a future revision, downloading a
package's source will fail using 'svn checkout -r'. Fix using
URL@REV syntax since the SVN command will then verify the path at the
specified version. Without using the URL@REV syntax, the SVN command
will attempt to verify the path exists on the the HEAD revision before
checking out the path at the specified revision.

Error seen if SVN path has been deleted in HEAD using svn checkout -r:
  svn: E160013: '/svn/path/' path not found

Signed-off-by: Ryan Barnett <ryan.barnett@rockwellcollins.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>