altoslib: Make AltosReplayReader start synchronously enough to track states

The Altos UI needs to see 'pad' state and then 'boost' state so that
it will automatically switch tabs during the flight. When reading from
eeprom files, the only way that is going to happen is if the reader
thread waits until the UI has definitely seen 'pad' state, which we do
by simply delaying the reader thread until after that has happened.

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