altos: Make telescience reliably log only when told to

Use a pull-up on telescience and tri-state input on telemetrum at boot
time. Enable logging by pulling the TM output low and enabling as an
output, disable by pulling the TM output high and switching back to
input mode.

Also, ignore pin state changes for 5 seconds at boot to give things a
chance  to settle down.

Should work when both boards are powered up together.

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