[stm32] add usb_tunnel, cleanup setup makefile

also replace usb_tunnel_0 and usb_tunnel_1 on lpc with one usb_tunnel target with configurable uart
[stm32] usb_serial: check if connected in putchar and send

cleanup and rename VCOM_transmit_message to VCOM_send_message
[setup] one usb_tunnel for both lpc21 and stm32
[led] add LED_AVAILABLE macro