batman-adv: Switch to HTTPS version of links

open-mesh.org and its subdomains can only be accessed via HTTPS. HTTP-only
requests are currently redirected automatically to HTTPS but references in
the source code should be only https.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>