Make RDF beacon only run on pad and after landing.

It's pretty much impossible to RDF the rocket during flight, and it
interferes with the telemetry data stream. Leave it enabled on the pad
so that radios can be tested, and then re-enable it once the rocket
has landed.

This patch also turns the rdf 'on' time into a parameter so it can be
changed, and then sets that parameter to 500ms, once every 5 seconds.

Signed-off-by: Keith Packard <keithp@keithp.com>