scripts/functions: svn retrieval first tries the mirror for tarballs

The svn download helper looks for the local tarballs dir to see if it
can find a pre-downloaded tarball, and if it does not find it, does
the actual fetch to upstream via svn.

In the process, it does not even try to get a tarball from the local
mirror, which can be useful if the mirror has been pre-populated
manually (or with a previously downloaded tree).

Fake a tarball get with the standard tarball-download helper, but
without specifying any upstream URL, which makes the helper directly
try the LAN mirror.

Of course, if no mirror is specified, no URL wil be available, and
the standard svn retrieval will kick in.

Reported-by: ANDY KENNEDY <ANDY.KENNEDY@adtran.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Bare metal config options

## select BARE_METAL

##

## help 'Bare metal' refers to those programs that run without any kernel.

## help

## help You probably want to say 'y' here if you plan to use your compiler

## help to build bootloaders. It is not yet suitable to build Linux kernels,

## help though, because the APCI stuff relies on the target C library headers

## help being available?!?!...