AP_Logger: fix locking issues, log wrapping and status messages

[andyp1per]

This is a follow-up to #13130
I was getting hangs in chip erase which lead to corrupted logs. This PR fixes this problem.
It also shortens some of the status messages so that they can be seen.

Tests done:

• Start and stop logs using LOG_DISARMED, check logs list ok
• Allow logs to wrap so that indexing no longer starts at 1, check logs list ok
• Allow a single log to wrap itself, check listing works ok and log can be downloaded
• Chip erase works and logging will work again afterwards
• Pull the power during chip erase and make sure it restarts
• Chip erase and then check that logs can be written once again without rebooting

All pass except the last one. So if you erase the chip you need to reboot to get logging to work again. This seems like a small inconvenience and nothing actually breaks in all these tests

[andyp1per]

I think there may still be a problem if your flight is long enough to wrap a single log, but not proved this yet.

[andyp1per]

@rmackay9 FYI

[andyp1per]

Ok, if I set log disarmed to 1 and just wait until the log wraps it's pretty easy to show the problems. I then end up with:

STABILIZE> Requesting log list
Log 65535  numLogs 3 lastLog 1 size 256
Log 0  numLogs 3 lastLog 1 size 0
Log 1  numLogs 3 lastLog 1 size 16711424

and maxproxy reports:

APM: No last page of log 65534 at top=8A21 or bot=8A20
APM: No last page of log 65535 at top=8A21 or bot=8A20

I will investigate more. Interestingly my checking code thinks everything is fine.

[andyp1per]

Ok, I have it. We weren't accounting for the fact that when we wrap a log we erase blocks as we go, so there is a deadzone between the end of the last log and the start of the first log that is filled with 0xFFFF which confuses all of the logic around finding log numbers and pages. The solution is to make sure we skip this deadzone when looking for the start of the first log. In @tridge 's defense I think that the original code may have had some allowance for this, but in fixing some of the other issues I clearly undid that logic. The PR now works with wrapped files.

[andyp1per]

@tridge I'm pretty confident in this now. I have created multiple logs via LOG_DISARMED and rebooting and then continuously done "log list" from mavlink with this on. This has correctly detected the wrap and then also has correctly detected the number of logs (reducing) as they gradually get overwritten. Pretty happy with this change now. @rmackay9 FYI.

[rmackay9]

FYI, this is included in Copter-4.0.2-rc4