batman-adv: beautify tt_global_add() argument list

Instead of adding a new bool argument each time it is needed, it is better (and
simpler) to pass an 8bit flag argument which contains all the needed flags

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>