printk: move console printing to kthreads

Create a kthread for each console to perform console printing. Now
all console printing is fully asynchronous except for the boot
console and when the kernel enters sync mode (and there are atomic
consoles available).

The console_lock() and console_unlock() functions now only do what
their name says... locking and unlocking of the console.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>