printk: nbcon: Assign nice -20 for printing threads

It is important that console printing threads are scheduled
shortly after a printk call and with generous runtime budgets.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-17-john.ogness@linutronix.de


Signed-off-by: Petr Mladek <pmladek@suse.com>