Allow disabling CDC with -DCDC_DISABLED

based on
https://github.com/gdsports/usb-metamorph/tree/master/USBSerPassThruLine

CDC_DISABLED is also used in ArduinoCore-samd for the same purpose.

You can emulate a USB Boot Keyboard with compatible Arduinos
(like the Leonardo or the SparkFun Pro Micro), but by default it won't
work with some hosts (definitely my Aten KVM switch, AFAIK also some
BIOSes, old Windows versions and some accessibility devices), because
they get confused by the "modem" and CDC devices used to provide a
serial console over USB.

This change makes it relatively easy to disable the CDC at compiletime.
Disabling it of course means that the serial console won't work anymore,
and that you can only flash directly after resetting the board.

See also NicoHood/HID#225 and
arduino/Arduino#6387 and
https://forum.arduino.cc/index.php?topic=545288.msg3717028#msg3717028

cores/arduino/CDC.cpp

@@ -22,6 +22,13 @@
 
 #if defined(USBCON)
 
+#ifndef CDC_ENABLED
+
+#warning "! Disabled serial console via USB (CDC)!"
+#warning "! With this change you'll have to use the Arduino's reset button/pin to flash (upload)!"
+
+#else // CDC not disabled
+
 typedef struct
 {
 	u32	dwDTERate;
@@ -299,4 +306,5 @@ int32_t Serial_::readBreak() {
 
 Serial_ Serial;
 
+#endif /* if defined(CDC_ENABLED) */
 #endif /* if defined(USBCON) */

cores/arduino/USBCore.cpp

@@ -69,8 +69,18 @@ const u8 STRING_MANUFACTURER[] PROGMEM = USB_MANUFACTURER;
 #define DEVICE_CLASS 0x02
 
 //	DEVICE DESCRIPTOR
+
+#ifdef CDC_ENABLED
 const DeviceDescriptor USB_DeviceDescriptorIAD =
 	D_DEVICE(0xEF,0x02,0x01,64,USB_VID,USB_PID,0x100,IMANUFACTURER,IPRODUCT,ISERIAL,1);
+#else // CDC_DISABLED
+// The default descriptor uses USB class OxEF, subclass 0x02 with protocol 1
+// which means "Interface Association Descriptor" - that's needed for the CDC,
+// but doesn't make much sense as a default for custom devices when CDC is disabled.
+// (0x00 means "Use class information in the Interface Descriptors" which should be generally ok)
+const DeviceDescriptor USB_DeviceDescriptorIAD =
+	D_DEVICE(0x00,0x00,0x00,64,USB_VID,USB_PID,0x100,IMANUFACTURER,IPRODUCT,ISERIAL,1);
+#endif
 
 //==================================================================
 //==================================================================
@@ -328,10 +338,12 @@ int USB_Send(u8 ep, const void* d, int len)
 u8 _initEndpoints[USB_ENDPOINTS] =
 {
 	0,                      // Control Endpoint
-
+
+#ifdef CDC_ENABLED
 	EP_TYPE_INTERRUPT_IN,   // CDC_ENDPOINT_ACM
 	EP_TYPE_BULK_OUT,       // CDC_ENDPOINT_OUT
 	EP_TYPE_BULK_IN,        // CDC_ENDPOINT_IN
+#endif
 
 	// Following endpoints are automatically initialized to 0
 };
@@ -373,10 +385,12 @@ void InitEndpoints()
 static
 bool ClassInterfaceRequest(USBSetup& setup)
 {
+#ifdef CDC_ENABLED
 	u8 i = setup.wIndex;
 
 	if (CDC_ACM_INTERFACE == i)
 		return CDC_Setup(setup);
+#endif
 
 #ifdef PLUGGABLE_USB_ENABLED
 	return PluggableUSB().setup(setup);
@@ -466,7 +480,9 @@ static u8 SendInterfaces()
 {
 	u8 interfaces = 0;
 
+#ifdef CDC_ENABLED
 	CDC_GetInterface(&interfaces);
+#endif
 
 #ifdef PLUGGABLE_USB_ENABLED
 	PluggableUSB().getInterface(&interfaces);

cores/arduino/USBDesc.h

@@ -26,8 +26,25 @@
 
 #define ISERIAL_MAX_LEN     20
 
+// Uncomment the following line or pass -DCDC_DISABLED to the compiler
+// to disable CDC (serial console via USB).
+// That's useful if you want to create an USB device (like an USB Boot Keyboard)
+// that works even with problematic devices (like KVM switches).
+// Keep in mind that with this change you'll have to use the Arduino's
+// reset button to be able to flash it.
+//#define CDC_DISABLED
+
+#ifndef CDC_DISABLED
+#define CDC_ENABLED
+#endif
+
+#ifdef CDC_ENABLED
 #define CDC_INTERFACE_COUNT	2
 #define CDC_ENPOINT_COUNT	3
+#else // CDC_DISABLED
+#define CDC_INTERFACE_COUNT	0
+#define CDC_ENPOINT_COUNT	0
+#endif
 
 #define CDC_ACM_INTERFACE	0	// CDC ACM
 #define CDC_DATA_INTERFACE	1	// CDC Data