altos: Require sequencing through 'main' state before landing

The old version of the code would permit the flight to go straight
from 'drogue' to 'landed' without passing through 'main' at all. This
meant that a false landing detection would leave the main charge
unfired, potentially causing the airframe to land on drogue alone.

Requiring that the flight sequence pass through main ensures that the
main charge will get fired at the right time, although if the airframe
lands higher than that altitude, it will not go to 'landed' mode ever.

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