wait_until_ready() polls MAVSDK's is_armable, but a known hysteresis gap
(and estimator heading-reference flicker at startup) means the flag can
read armable while PX4 then denies the arm command with 'no heading
reference'. This made arm() fail intermittently. Retry the arm command
until the autopilot accepts it, so the transient is absorbed while real
failures still time out.
Signed-off-by: Julian Oes <julian@oes.ch>