altos: Eliminate height requirement for coast detect

We had required a minimum altitude of 100m to transition from boost to
coast. With small motors in a heavy multi-staged rocket, this can fail
to detect coast in time to light the second motor.

Also, this would fail to deploy recovery systems if the flight failed
before reaching 100m.

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