In the general case you can't - because a console may not have a tty. Quite a few don't. For the cases where one has a device I can't see a clean way to do it as an fstat on the console device will give you the console major/mninor and some code depends upon that.

The iniability to get it is clearly a flaw and you need the sysfs link or a sysfs list of consoles (again remembering many of them won't have a tty bound).

For init it is /dev/console as opened at boot time and passed to the initial user init task but with no nice way to work back to a device node.

Incidentally the primary console (as in printk) can be accessed as a tty via the ttyprintk driver which got added recently because embedded people wanted to use the printk interface as a (write only) tty console as well.