halium-overlay: Add NetworkManager config to fix hotspot passwords

Password protected hotspot currently starts but attempting to connect to it simply fails on UT 20.04.

Fixes volla-yggdrasil#7 (closed).

I'm not the biggest fan of overlaying default-wifi-powersave-on.conf but there isn't an empty e.g. halium.conf one to overlay for the legacy style full systemimage ports.

@peat-psuwit What would you think of an empty overlayable /etc/NetworkManager/conf.d/halium.conf perhaps with a # comment?