Dynamic deletion of log messages

[Friedjof]

The approaches to this can already be found in the LoggingManager under delete_first_n_lines.
The procedure is as follows:

1. a temporary file is created in "w" mode.
2. the system.log is scanned up to the new first line.
3. transfer to the new file is started.
4. the system.log is deleted and the temporary file is renamed.

An error occurs here during the last step. It seems that system.log is still open. However, I am very sure that this is not the case.
I have already tried to do the whole thing without a temporary file using a string buffer. However, that did not work either. I don't know exactly but I can't seem to write anything to the file. It would be good to start at that point.

For debugging, you can proceed as follows:

1. make start upload fs and flash code
2. connect to the wifi
3. via https://192.168.4.1/logging you can read the content of the system.log.
4. you can find more information about the length of the file and a bit more at https://192.168.4.1/log_lines.

This last endpoint can be removed after the fix.