Refactored cdcacm_init into a module and added a paramter to allow always starting mavlink on
    USB, also added a paramter to choose the mode. The current default behavior is to wait and listen
    for data on USB and auto-detect the protocol (mavlink, nsh, ublox). This results in the mavlink
    stream not starting until something else on the mavlink network sends a packet first. The new
    default behavior is to always start mavlink.

    Added parameters
    MAV_USB_ENABLE -- default 1 (always start mavlink on USB)
    MAV_USE_MODE -- default 3 (onboard)

…ore sercon

…s with CONFIG_CDCACM in their nsh/defconfig