printk: Provide threadprintk boot argument

For PREEMPT_RT, legacy console printing is performed in a dedicated
kthread. However, this behavior can also be interesting for other
preemption models as it minimizes the duration of printk() calls by
deferring all printing.

Provide a new boot argument "threadprintk" that will create the
dedicated kthread for legacy console printing for !PREEMPT_RT
systems.

The implementation is the same as the "threadirqs" boot argument.

Users should be aware that if this option is enabled, the shutdown,
reboot, and panic messages probably will not be visible on the
legacy consoles.

Non-legacy consoles (NBCON) already have their own dedicated kernel
threads for printing and reliable shutdown, reboot, and panic
printing. This option really only applies to legacy consoles.

Users can view /proc/consoles to see if their console driver is
legacy or not. NBCON console drivers are shown with 'N'.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>