[mavlink] Parameter to always start on USB #21541

Job	Run time
build (check_format)	1m 6s
build (tests)	5m 31s
build (tests_coverage)	6m 21s
build (px4_fmu-v2_default stack_check)	3m 19s
build (validate_module_configs)	52s
build (shellcheck_all)	1m 43s
build (NO_NINJA_BUILD=1 px4_fmu-v5_default)	4m 38s
build (NO_NINJA_BUILD=1 px4_sitl_default)	3m 11s
build (airframe_metadata)	1m 27s
build (module_documentation)	1m 5s
build (parameters_metadata)	1m 25s
	30m 38s

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