boottrace: Fix tracing the execution of SYSINITs

[zlei]

Summary

For SYSINITs, boottrace should log the done event, aka after executing
a group of functions in a subsystem.

For dynamically loaded modules, at least a start event should be logged,
so that we can get precise delta.

Fixes: 5a8fceb3bd9f boottrace: trace annotations for startup and shutdown
MFC after: 1 week