wifi: Allow building without Wifi HAL

 * For example devices with older broadcom chips with driver built
   as module doesn't like this. When the device goes into suspend
   mode, the wifi service crashes and the device soft reboots.

 * To not build the HAL, use:
   BOARD_NO_WIFI_HAL := true

Change-Id: Ia186ce3a59a082cfe98d283492fa450014af52f4