batman-adv: improve the TT component to support runtime flag changes

Some flags (i.e. the WIFI flag) may change after that the
related client has already been announced. However it is
useful to informa the rest of the network about this change.

Add a runtime-flag-switch detection mechanism and
re-announce the related TT entry to advertise the new flag
value.

This mechanism can be easily exploited by future flags that
may need the same treatment.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>