Like git which can have submodules, subversion can have externals. The
default behaviour for subversion is to retrieve all the externals,
unless told otherwise.

For some repositories, the externals may be huge (e.g. a dataset or some
assets) and may not be required for building the package. In such a
case, retrieving the externals is both a waste of network bandwitdh and
time, and a waste of disk storage.

Like for git submodules and git lfs, add an option that packages can set
to specify whether they want externals or not.

Since we've so far been retrieving externals, we keep that the default,
and packages can opt-out (rather than the opt-in for git submodules or
git lfs).

We must only set it when the package is actually hosted on svn, to avoid
passing -r when the package is not hosted by svn; otherwise, -r would
also be passed e.g. to a git-hosted package, triggering the download of
git submodules even when they are not requested. We need to do so,
because we have a default value, which we usually do not have in other
download options.

Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>