RPi3: set up WiFi to act as client and access point simultaneously

[jbrzusto]

use cases:

• set up and check access to a WiFi router on the SGPi while connected to the SGPi web interface via WiFi

• remote troubleshooting or data upload via phone tethering:

  • user connects to SGPi WiFi AP using phone
  • select "tether" from web interface, then enters essid, passphrase, and hits "Go"
  • user sets up phone as WiFi hotspot