Record average accelerometer value in flight start log record

Record average accelerometer value in flight start log record

The average accelerometer value cannot be extracted from the log as the
record starts after that is computed. As that drives much of the
accelerometer-based state transition logic, it is an important value to
have, so we log it as part of the flight start record now.

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