Add symbolic names for USB Device String Descriptors within the demos, applications and bootloaders.

Demos/Device/ClassDriver/AudioOutput/Descriptors.c

@@ -57,8 +57,8 @@ const USB_Descriptor_Device_t PROGMEM DeviceDescriptor =
 	.ProductID              = 0x2046,
 	.ReleaseNumber          = VERSION_BCD(00.02),
 
-	.ManufacturerStrIndex   = 0x01,
-	.ProductStrIndex        = 0x02,
+	.ManufacturerStrIndex   = STRING_ID_Manufacturer,
+	.ProductStrIndex        = STRING_ID_Product,
 	.SerialNumStrIndex      = NO_DESCRIPTOR,
 
 	.NumberOfConfigurations = FIXED_NUM_CONFIGURATIONS
@@ -304,15 +304,15 @@ uint16_t CALLBACK_USB_GetDescriptor(const uint16_t wValue,
 		case DTYPE_String:
 			switch (DescriptorNumber)
 			{
-				case 0x00:
+				case STRING_ID_Language:
 					Address = &LanguageString;
 					Size    = pgm_read_byte(&LanguageString.Header.Size);
 					break;
-				case 0x01:
+				case STRING_ID_Manufacturer:
 					Address = &ManufacturerString;
 					Size    = pgm_read_byte(&ManufacturerString.Header.Size);
 					break;
-				case 0x02:
+				case STRING_ID_Product:
 					Address = &ProductString;
 					Size    = pgm_read_byte(&ProductString.Header.Size);
 					break;

Demos/Device/ClassDriver/AudioOutput/Descriptors.h

@@ -75,6 +75,17 @@
 			USB_Audio_Descriptor_StreamEndpoint_Spc_t Audio_StreamEndpoint_SPC;
 		} USB_Descriptor_Configuration_t;
 
+		/** Enum for the device string descriptor IDs within the device. Each string descriptor should
+		 *  have a unique ID index associated with it, which can be used to refer to the string from
+		 *  other descriptors.
+		 */
+		enum StringDescriptors_t
+		{
+		    STRING_ID_Language      = 0, /**< Supported Languages string descriptor ID (must be zero) */
+		    STRING_ID_Manufacturer  = 1, /**< Manufacturer string ID */
+		    STRING_ID_Product       = 2, /**< Product string ID */
+		};
+
 	/* Function Prototypes: */
 		uint16_t CALLBACK_USB_GetDescriptor(const uint16_t wValue,
 		                                    const uint8_t wIndex,