Bluetooth support on RPi Zero W disabled

It does not work on the RPi Zero W. Because Bluetooth support in Circle
is only rudimentary and cannot be extended because of legal reasons,
this will not be fixed.

Issue #90
Reported by @moahrs

lib/bt/btsubsystem.cpp

@@ -2,7 +2,7 @@
 // btsubsystem.cpp
 //
 // Circle - A C++ bare metal environment for Raspberry Pi
-// Copyright (C) 2015-2017  R. Stange <rsta2@o2online.de>
+// Copyright (C) 2015-2018  R. Stange <rsta2@o2online.de>
 // 
 // This program is free software: you can redistribute it and/or modify
 // it under the terms of the GNU General Public License as published by
@@ -40,12 +40,11 @@ CBTSubSystem::~CBTSubSystem (void)
 
 boolean CBTSubSystem::Initialize (void)
 {
-	// if USB transport not available, UART still free and this is a RPi 3B or Zero W:
+	// if USB transport not available, UART still free and this is a RPi 3B:
 	//	use UART transport
 	if (   CDeviceNameService::Get ()->GetDevice ("ubt1", FALSE) == 0
 	    && CDeviceNameService::Get ()->GetDevice ("ttyS1", FALSE) == 0
-	    && (   CMachineInfo::Get ()->GetMachineModel () == MachineModel3B
-		|| CMachineInfo::Get ()->GetMachineModel () == MachineModelZeroW))
+	    && CMachineInfo::Get ()->GetMachineModel () == MachineModel3B)
 	{
 		assert (m_pUARTTransport == 0);
 		assert (m_pInterruptSystem != 0);

sample/22-btsimple/README

@@ -1,6 +1,6 @@
 README
 
-You need a Raspberry Pi 3 (with internal Bluetooth device) or an external USB Bluetooth BR/EDR dongle to use this sample. Unfortunately I cannot tell which dongles will work because there are many different out there. The Bluetooth dongle has to be connected to an USB port of your Raspberry Pi.
+You need a Raspberry Pi 3 Model B (with internal Bluetooth device) or an external USB Bluetooth BR/EDR dongle to use this sample. Unfortunately I cannot tell which dongles will work because there are many different out there. The Bluetooth dongle has to be connected to an USB port of your Raspberry Pi.
 
 After switching on the Raspberry Pi the Act LED blinks 5 times to show the image was loaded right. When the system initialization is over (see notice below) the sample program does a Bluetooth Inquiry operation for 20 seconds to find all visible Bluetooth devices. Displaying the results (address, class code and name of each device) may take some additional seconds because the names of the found devices have to be requested.