uts: kmdb should use "diag-device" when set
diag-device boot property is set, kmdb should appear on the diag device instead of the regular console -- even after the loading of the console drivers disables early boot mirroring. This enables the engineer to use the serial port for the debugger while the regular console is on the framebuffer, enabling debugging of the framebuffer console software itself.
This change also documents the
diag-device property in