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

[Julius Werner (Code Review)]

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

Change subject: lib: Add a mutex
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Patch Set 11: Code-Review+1

(3 comments)

Patchset:

PS11: 
FWIW this pretty much looks good to me now, but let's way for Kyösti to have another chance to comment as well.

File src/include/mutex.h:

https://review.coreboot.org/c/coreboot/+/59320/comment/474f2f96_ecd463f5 
PS11, Line 12: static inline void mutex_lock(struct mutex *mutex);
nit: a few lines of documentation would be nice.

File src/lib/mutex.c:

https://review.coreboot.org/c/coreboot/+/59320/comment/0d13877b_5ef78514 
PS11, Line 43: __atomic_load_1(&mutex->locked, __ATOMIC_RELAXED)
_mutex_is_locked_smp()?