Mavlink rates

This PR is WIP. Adds a decent method of printing the radio status.
Have you seen that I used the actual message size here before? Is there any particular reason to not use it, given that we know it?
The proper approach would be to give the mavlink_pm_queued_send() the free buffer space and let its internal logic decide if it can send. As the messages are exactly known at that level, this will increase the throughput, provide a better timing and avoid magic values.