Follows the rename of AP_ADSB_uAvionix_MAVLink to AP_ADSB_MAVLink and of
HAL_ADSB_UAVIONIX_MAVLINK_ENABLED to HAL_ADSB_MAVLINK_ENABLED.
Spelled ADSB_MAVLINK rather than ADSB_MAVLink to match the options either
side of it - ADSB_SAGETECH, ADSB_SAGETECH_MXS and ADSB_UAVIONX_UCP.
Note this changes a user-visible option name: --disable-ADSB_UAVIONIX now
fails with waf's "no such option" rather than silently building a feature
the user asked to leave out.