printk: Add non-BKL (nbcon) console basic infrastructure

printk: Add non-BKL (nbcon) console basic infrastructure

The current console/printk subsystem is protected by a Big Kernel Lock,
(aka console_lock) which has ill defined semantics and is more or less
stateless. This puts severe limitations on the console subsystem and
makes forced takeover and output in emergency and panic situations a
fragile endeavour that is based on try and pray.

The goal of non-BKL (nbcon) consoles is to break out of the console lock
jail and to provide a new infrastructure that avoids the pitfalls and
also allows console drivers to be gradually converted over.

The proposed infrastructure aims for the following properties:

  - Per console locking instead of global locking
  - Per console state that allows to make informed decisions
  - Stateful handover and takeover

As a first step, state is added to struct console. The per console state
is an atomic_t using a 32bit bit field.

Reserve state bits, which will be populated later in the series. Wire
it up into the console register/unregister functionality.

It was decided to use a bitfield because using a plain u32 with
mask/shift operations resulted in uncomprehensible code.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230916192007.608398-2-john.ogness@linutronix.de