printk: Bring back the RT bits.

This is a revert of the commits:
| 07a22b61 Revert "printk: add functions to prefer direct printing"
| 5831788a Revert "printk: add kthread console printers"
| 2d9ef940 Revert "printk: extend console_lock for per-console locking"
| 007eeab7 Revert "printk: remove @console_locked"
| 05c96b37 Revert "printk: Block console kthreads when direct printing will be required"
| 20fb0c82

 Revert "printk: Wait for the global console lock when the system is going down"

which is needed for the atomic consoles which are used on PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>