Report the kernel console on the boot screen

Report what console the boot loader is telling the kernel to use:
o Dual (Serial Primary)
o Dual (Video Primary)
o Serial
o Video

This allows the user to interrupt the boot and tweak the cosnole, if
needed, in a trivial way. Useful for installs where the default
selected may not be quite what you want, or when you are running a
dual setup and need to toggle over to the other console being primary.
The 'c'/'C' keys will do the cycling through the consoles. Note:
you'll still have to drop into the loader to set details about serial
consoles. And this doesn't change the console the loader is using.

Reviewed by: kevans@
MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D26573