fw/b: Add capability to allow tethering to use VPN upstreams

* Toggled on/off at runtime via a new hotspot lineage setting.

* Dynamically updates the tethering upstream for existing hotspot
  clients as VPNs are brought up / down or the hotspot setting
  is changed.

* This implementation depends on fw/b config_tether_upstream_automatic
  being set to true.

Change-Id: I2ac0b4acc0ea686dfdf54561cb3428808e337160