altosuilib: Zap all flash when upgrading TeleGPS from pre-1.9.7

Old versions of TeleGPS firmware would end up spraying log data all
over flash as they mis-computed the place to append new log data.

When the right hardware is detected, a warning dialog will pop up and,
if agreed to, the log storage flash will be completely erased before
the firmware upgrade happens.

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