* usb: Added parameter to enable always starting mavlink on USB.

    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)

* added 3 retries for opening serial port in mavlink, removed sleep before sercon

* added DRIVERS_CDCACM_AUTOSTART to ark-v6x default.px4board

* added CONFIG_DRIVERS_CDCACM_AUTOSTART=y to default.px4board for boards with CONFIG_CDCACM in their nsh/defconfig

* format

* remove PGA460 from COMMON_DISTANCE_SENSOR to save flash

* remove LIS2MDL from COMMON_MAGNETOMETER to save flash

* disable CONFIG_DRIVERS_CDCACM_AUTOSTART for fmu-v5 protected.px4board

* moved and renamed parameters, removed mode logic in mavlink

* changed parameter names, added mode none

* remove parameters from mavlink