altos: Handle flight_log_max not being multiple of ao_storage_block

altos: Handle flight_log_max not being multiple of ao_storage_block

In this case, flight records start in the middle of an erase block, but
now end at the erase block before the next flight.

When checking for an empty log slot, the entire erase block containing
the start of the flight is checked to make sure it's clear, skipping
it if not.

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