base-files: add 'isup' to the wifi script

base-files: add 'isup' to the wifi script

This is a silent command that allows easy wifi up/down automation for
scripts.

It takes one or multiple devices as arguments (or all if none are passed),
and the exit code indicates if any of those is not up.

E.g.:
wifi isup && echo "all wifi devices are up"
wifi isup radio0 || echo "this wifi is down"

Signed-off-by: Andre Heider <a.heider@gmail.com>