batman-adv: Use string choice helper to print booleans

The commit ea4692c7 ("lib/string_helpers: Consolidate string helpers
implementation") introduced common helpers to print string representations
of boolean helpers. These are supposed to be used instead of open coded
versions.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>