AP_Scripting: add binding for GCS last seen time

[IamPete1]

This adds a binding for sysid_myggcs_last_seen_time_ms renaming it to last_seen such then it can be called as gcs:last_seen().

This allows GCS failsafe actions to be implemented in scripting. Very basic example:

function update()

  local gcs_dt = millis() - gcs:last_seen()

  gcs:send_text(0, string.format("Last GCS was: %0.2f seconds ago", gcs_dt:tofloat() * 0.001))

  return update, 1000
end

return update()