When the @diag-device@ boot diag-device property is set, kmdb should appear on set up the diag device instead of the regular console -- even after the loading of the console drivers disables early boot mirroring. This enables the engineer variables to make it possible to use the serial port device for input and output. This will complement the debugger while the regular console is on the framebuffer, enabling debugging list of the framebuffer console software itself. This change also documents the @diag-device@ property in @eeprom(1M)@. existing (console related) input and output variables.