Extract interface numbers into enums.

Bootloaders/MassStorage/BootloaderMassStorage.c

@@ -44,7 +44,7 @@ USB_ClassInfo_MS_Device_t Disk_MS_Interface =
 	{
 		.Config =
 			{
-				.InterfaceNumber           = 0,
+				.InterfaceNumber           = INTERFACE_ID_MassStorage,
 				.DataINEndpoint            =
 					{
 						.Address           = MASS_STORAGE_IN_EPADDR,

Bootloaders/MassStorage/Descriptors.c

@@ -91,7 +91,7 @@ const USB_Descriptor_Configuration_t ConfigurationDescriptor =
 		{
 			.Header                 = {.Size = sizeof(USB_Descriptor_Interface_t), .Type = DTYPE_Interface},
 
-			.InterfaceNumber        = 0,
+			.InterfaceNumber        = INTERFACE_ID_MassStorage,
 			.AlternateSetting       = 0,
 
 			.TotalEndpoints         = 2,

Bootloaders/MassStorage/Descriptors.h

@@ -68,6 +68,16 @@
 			USB_Descriptor_Endpoint_t             MS_DataOutEndpoint;
 		} USB_Descriptor_Configuration_t;
 
+
+		/** Enum for the device interface descriptor IDs within the device. Each interface descriptor
+		 *  should have a unique ID index associated with it, which can be used to refer to the
+		 *  interface from other descriptors.
+		 */
+		enum InterfaceDescriptors_t
+		{
+			INTERFACE_ID_MassStorage = 0, /**< Mass storage interface descriptor ID */
+		};
+
 	/* Function Prototypes: */
 		uint16_t CALLBACK_USB_GetDescriptor(const uint16_t wValue,
 		                                    const uint8_t wIndex,