Change in coreboot[master]: lib: Add a mutex

[Raul Rangel (Code Review)]

Attention is currently required from: Julius Werner, Rob Barnes, Kyösti Mälkki, Karthik Ramasubramanian.
Raul Rangel has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/59320 )

Change subject: lib: Add a mutex
......................................................................

Patch Set 3:

(2 comments)

File src/include/mutex.h:

PS2:
> License

Done

https://review.coreboot.org/c/coreboot/+/59320/comment/9807d471_40f9f83d 
PS2, Line 11: void mutex_unlock(struct mutex *mutex);
> These should be static inline no-ops when neither SMP nor threading is enabled.

So I wanted to make it so the lock/unlock asserts were checked regardless of threading or SMP. This way we don't introduce code that works with coop disabled, but then breaks when coop is enabled. I felt the increase in BSS and code was small enough that it was worth it. What do you think?

Otherwise I can add a check into the functions:

i.e.,
```
    if (ENV_STAGE_SUPPORTS_SMP)
    	mutex_lock_smp(mutex);
    else if (CONFIG(COOP_MULTITASKING))
    	mutex_lock_nosmp(mutex);
```