logging: fs backend: optional append to latest file

Make appending to the newest log file in the fs logging backend optional.
By default, if there is still free space in the latest log file,
the fs logging backend appends to it on startup. This is useful for saving
space and avoiding the removal of older log files, if the maximum number
of log files has been reached. The drawback of this behavior is,
that log files that got appended can not be decoded, if the firmware
has changed between startups, for instance by an update,
since the log_dictionary.json used for decoding has also changed.
Therefore, it may be desirable to deactivate appending to log files.

Signed-off-by: Niklas Fabian <niklas.fabian@lemonbeat.com>