If we take platform as a component, platform/source would be outside of this component.
I removed the prefix as these are implementation specific (they are located in source folder,
not in include as the others).
Chrono conversions inadvertantly changed the core timed sleep routine
used by the RTOS idle to use `OsTimer::update_and_get_tick()` instead of
`OsTimer::get_tick()`.
Correct this, and expand/clarify documentation and naming to try to
prevent recurrence.
Another minor fix observed while inspecting code - `OsClock` can't just
use `milliseconds`, it should match the period of `OsTimer`, which
theoretically can be different.