Add USB keyboard only option

teensy3/usb_desc.h

   #define ENDPOINT3_CONFIG	ENDPOINT_RECEIVE_ONLY
   #define ENDPOINT4_CONFIG	ENDPOINT_TRANSIMIT_ONLY
 
+#elif defined(USB_KEYBOARDONLY)
+  #define VENDOR_ID		0x16C0
+  #define PRODUCT_ID		0x0482
+  #define MANUFACTURER_NAME	{'T','e','e','n','s','y','d','u','i','n','o'}
+  #define MANUFACTURER_NAME_LEN	11
+  #define PRODUCT_NAME		{'K','e','y','b','o','a','r','d'}
+  #define PRODUCT_NAME_LEN	8
+  #define EP0_SIZE		64
+  #define NUM_ENDPOINTS         4
+  #define NUM_USB_BUFFERS	14
+  #define NUM_INTERFACE		3
+  #define SEREMU_INTERFACE      1	// Serial emulation
+  #define SEREMU_TX_ENDPOINT    1
+  #define SEREMU_TX_SIZE        64
+  #define SEREMU_TX_INTERVAL    1
+  #define SEREMU_RX_ENDPOINT    2
+  #define SEREMU_RX_SIZE        32
+  #define SEREMU_RX_INTERVAL    2
+  #define KEYBOARD_INTERFACE    0	// Keyboard
+  #define KEYBOARD_ENDPOINT     3
+  #define KEYBOARD_SIZE         8
+  #define KEYBOARD_INTERVAL     1
+  #define KEYMEDIA_INTERFACE    2	// Keyboard Media Keys
+  #define KEYMEDIA_ENDPOINT     4
+  #define KEYMEDIA_SIZE         8
+  #define KEYMEDIA_INTERVAL     4
+  #define ENDPOINT1_CONFIG	ENDPOINT_TRANSIMIT_ONLY
+  #define ENDPOINT2_CONFIG	ENDPOINT_RECEIVE_ONLY
+  #define ENDPOINT3_CONFIG	ENDPOINT_TRANSIMIT_ONLY
+  #define ENDPOINT4_CONFIG	ENDPOINT_TRANSIMIT_ONLY
+  #define ENDPOINT5_CONFIG	ENDPOINT_TRANSIMIT_ONLY
+  #define ENDPOINT6_CONFIG	ENDPOINT_TRANSIMIT_ONLY
+
 #elif defined(USB_HID)
   #define VENDOR_ID		0x16C0
   #define PRODUCT_ID		0x0482