serial console: allow for ditto outputs on screen and serial consoles
Currently only one output device can be used by OI to report on progress in system booting, output messages written to /dev/console, etc. For example, if the default console is set to serial and virtual terminal daemons are not enabled, the screen remains blank and gives no indication whether the system has booted or is still working; the physical teerminal also can not be used for system boot repairs in this case.
This RFE proposes to add a mechanism to request a secondary console (as a kernel option and/or eeprom=bootenv.rc setting), so that at least important messages (the OS version header, the boot errors leading to requests for administrative/single-user mode logins), or perhaps all messages for /dev/console, are output not only to the default console device, but also to the other requested device. A model to mirror might be the Linux kernel boot options (switching physical consoles, serial consoles and net_consoles). See also RFE #2878 regarding the simultaneous login functionality on both types of interactive consoles.