Add keyboard and multimedia support with serial

[rastersoft]

The original, Atmega32U4 based, teensy, doesn't support serial port emulation and multimedia keycodes support at the same time because the microcontroller doesn't have enough endpoints.

This patch adds an extra option where the joystick is replaced by the multimedia keys support, thus allowing to have full keyboard/mouse support along with full serial port support.

To enable it, just define USB_SERIAL_HID_NO_JOYSTICK.