this doesn't actually allow you to specify a default, despite the name
and a couple of hwdefs trying to use it that way.
This would have stopped being useful when we did the
compass-prority-table stuff - or before.
fixes#19765, an alternative to #19768
the advantage of this approach is it is less error prone, as the
actual position and size of the storage sectors is calculated
Andy Piper
Peter Barker
Andrew Tridgell