See drivers/char/vt.c.

'console_driver' is defined only when CONFIG_VT_CONSOLE is set.
However it is used by vty_init() which is outside the scope
of CONFIG_VT_CONSOLE.

I think the fix is to move console_driver definition outside
CONFIG_VT_CONSOLE.  See the attachment.  Hopefully someone with
more knowledge on this can validate that.

Jun