* Add Corne and Ferris shared layouts, and update the respective
shields to use them.
* Add a sample Hummingbird physical layout for testing posix xiao.
* Add Sofle physical layout as an additional reference.
Removed "label" properties which no longer have any function.
Labels are still used as layer names and as identifiers for sending
behaviors between sides of a split keyboard, so those have been left
alone for now.
Aligns *.h and *.c to underscore naming convention.
These were kept (with warnings) for backwards compatibility with external boards/shields:
- kscan-mock.h
- matrix-transform.h
They should be removed in the future.
PR: #523
