bsp/default: nodewatcher: use bridge from iproute2

brctl isn't part of OpenWRT anymore. Rewrite the client-count to bridge util
from iproute2 package.

Another alternative may be to use "batctl tl" but there we get false positives
for local interfaces.

Signed-off-by: Tim Niemeyer <tim.niemeyer@mastersword.de>
Reviewed-by: Steffen Pankratz <kratz00@gmx.de>