launchs wpa manually (as if it were launched by ifup if interface wpa setup
was done in /network/interfaces)
/sbin/wpa_supplicant -s -B -P /run/wpa_supplicant.wlan0.pid -i wlan0 -D nl80211,wext -C /run/wpa_supplicant
options = {
interface: 'wlan0',
drivers: [ 'nl80211', 'wext' ]
}
function manual(options, callback) {
  var command = [
    'wpa_supplicant',
    '-i', options.interface,
    '-s -B -P /run/wpa_supplicant/' + options.interface + '.pid',
    '-D', options.drivers.join(','),
    '-C /run/wpa_supplicant'
  ].join(' ');

  return this.exec(command, callback);
}