Currently mosys has hardcoded limits for eventlog full and shrink sizes. The "full" size is 3/4, which makes some sense if the log region is small, like 1 4KiB sector.

However, on systems with more space dedicated to the eventlog the 3/4 "full" threshold might be considered wasteful and preserving 1/4 old entries might be considered excessive.

We should probably add a member to the eventlog_cb to get the desired thresholds, and can fall back to default values if none are specified.