Linaro user needs to be in netdev group

After installing the #933 build I was not able to select a wifi connection through the GUI because of a permissions issue. I discovered that the linaro user needed to be added to the netdev group for the UI based network selection to work correctly.

@alimon, are you aware about that?

@Loic @mcharleb The linaro user is configured to be in netdev group when Debian image is created [1]. Occasionally at first boot/login permissions wasn’t set properly but looks like a bug in the login manager instead of not being in netdev.

[1] https://git.linaro.org/ci/fai.git/tree/scripts/LINARO/02-add_linaro_to_groups.chroot#n4