swig: add swig2.0 -> swig symlink

swig: add swig2.0 -> swig symlink

The cmake detection script provided with cmake will first look for a
binary called swig2.0 then for a binary called swig.

host-swig only installs a binary called swig, but if the host
distribution has instaled a binary called swig2.0, it will be
preferred over our swig, which isn't great. This patch creates a
symlink swig2.0 -> swig to prevent this from happening.

[Thomas: add comment in the code and improve the commit log]

Signed-off-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>