swig: use a relative path for the host swig2.0 symlink's target

An absolute path isn't really necessary and breaks relocating

Signed-off-by: Phil Eichinger <phil@zankapfel.net>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>