consoles: polling support, kgdboc

consoles: polling support, kgdboc

polled console handling support, to access a console in an irq-less
way while in debug or irq context.

absolutely zero impact as long as CONFIG_CONSOLE_POLL is disabled.
(which is the default)

[ [email protected]: lots of cleanups ]
[ [email protected]: redesign, splitups, cleanups. ]

Signed-off-by: Jason Wessel <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Jan Kiszka <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>